Diluting viscosity is an important form for heavy oil recovery technology, while mixer is the main tool to improve mixing effect in this process. Firstly, a new type of core tube heavy oil mixing and diluting mixer structure was proposed. The method of core pipe micro hole ejection was adopted to realize the mixing of thin oil and heavy oil in annular space, meanwhile, the tapered tube structure was designed to form irregular turbulence according to the Venturi effect to achieve twice mixing. The proposed structure had a good mixing effect and little influence on oil hydraulic pressure drop. Secondly, the influence of five factors which included micro pore diameter, micro pore density, injection angle, internal cone angle and inlet and outlet pressure difference on the mixing non-uniformity coefficient and the dilution ratio was studied by CFD (computational fluid dynamics)numerical simulation. Finally, a mixer for butt 3 1/2" tubing was designed according to the actual conditions of heavy oil mixing and diluting in Tahe oilfield and the laboratory experiment was carried out. The obtained mixing non-uniformity coefficient was 0.023 4 and the dilution ratio was 0.290 1, which was achieved the ideal level and indicated that this mixer has a good diluting viscosity performance. The research results provide technical support for improving the recovery efficiency of heavy oil and reduce the failure rate of pumping equipment.