Many of the productive and daily activities of people are done in the sitting position, so it is important to study posture comfort. As sitting comfort in the seat design plays a more and more important role, the studies of the seat comfort gradually transform into the evaluation of sitting comfort. In order to achieve the demand of the human body comfortable sitting posture,the design of humanized seat must ensure that its comfort level as high as possible. With the purpose of improving the seat comfort which provided to the body, there had been much previous research on measurements of comfort and discomfort in sitting,sitting posture features extraction analysis was an important issue in the product design. In view of the sitting comfort evaluation,the method which integrated classical optical motion capture technology and new investigation methods in real environment. In the experiments, Motion Analysis Eagle Digital System was used to capture the body motion of 40 participants, and obtained the human body each joint point in the space. During the 11 Eagle cameras setup was completed, a self developed MATLAB program to analysis and calculate the data captured was used. Some particular features were obtained by principal component analysis (PCA). Moreover, the distinct difference between ergo chair and standard chair on the joint angle and the torque was obtained. After checking all the angles' and torques' appearance, based on the theory of ergonomics, the angles of the trunk, thighs, knees were identified as important features for sitting posture recognition. It provides a theoretical basis for seat design. The results showed that this digital evaluation of sitting posture method integrates Chair Study Questionnaire, which provides an effective method to evaluate the comfort of the seat.