Family planning policy implemented over 30 years has generated a lot of only-children in China， and it has become a hot issue. The government loosened its single-child policy to allow a parent who has an only-child in their family to have another baby in 2014. Definitely this change will have an impact on the size and structure of the only-children. Thus， it is worth studying and forecasting under the new policy， especially in the context of ″less younger children and more aging population，″ so as to provide the basic information for the policy-maker to develop one-child parents pension policies in China． Based on 1% national population sample survey in 2005 and census data in 2010， we estimate the multi-age marriage probability among couples of double only-children， single only-children and non-only children， by following the idea and method of parity progression to measure out different number of couples who will be allowed to give only-children， or second children under the new fertility policy， and based on the number of second children and the parity-specific births to adjust dynamically the attributes of only children or non-only children， and to estimate the number of age-sex-specific only-children． The results show that by the end of 2013， the number of only-children had reached about 218 million， of which 69.0% lived in cities or towns， and 31.0 % in rural areas. With the implementation of the new fertility policy in 2014， the number of only-children will continue to grow， and the long term trend of lower birth rate is immutable. The number of only-children will reach 303 million by 2050， but 45 million less than under the one-child policy， which means less of only-child risky families and a more harmonious family structure conducive to future pension protection.  The spatial distribution of only-children is heavier in our urban areas and eastern regions than other areas. There will be 303 million only-children in 2050， of which those in urban areas will account for 89.1% and the rural only-children for 10.9%. The only-children are mainly concentrated in the eastern China， followed by the Middle and West regions， and fewest in northeast China. However， the proportion of only-children in the eastern region will increase before decreasing， rise slowly in the Middle and West regions， and show a steady decline in the Northeast region. The disparity in provincial distribution of only-children is quite large， with over 10 million each existing in 12 provinces such as Jiangsu， Shandong but less than one million each in Tibet， Qinghai， and Hainan.  The age structure of the only-children is dominated by those under 40 years of age at present， among whom the proportion of children aged 0-14 will show a rapid decline before a stable situation， the young and prime adults aged 15-39 will show a slow increase before a quick decrease， while the old adults aged 40 and above will rise continuously. The only-children aged 40 years and above will gradually replace the young in 2040 as the main body of only-children. As they grow old， the risk of death will increase， and the proportion of one-child parents who lose their only-child will increase too. Since the pension of the parents of only-children and dead only-children will become serious challenges， we should be prepared to deal with their social security in advance． The fertility policy simulation technology is used here， together with marriage probability of multi-age difference between couples， parity progression ratio and dynamic adjustment of the characteristics of only-children. The provincial-level forecast of the number of only-children under new fertility policy is conducted for the first time to provide the size and age structure of the only-children on the national， regional and provincial levels， in urban and rural areas. It should be noted that the statistical standard of only-children as well as the data are forecast under the condition that the current fertility patterns are unchanged， and the death probability of only-children is the same as non-only children．