To increase the power generation efficiency of tower solar thermal power generation system, an annular compound parabolic concentrator is designed and installed at the heat receiver which is on the top of this system. The structure of the annular compound parabolic concentrator can well adapt to the light field distribution of the annular heliostat field in the system, and the solar energy reflected by heliostats can be efficiently concentrated by annular compound parabolic concentrator to the surface of the heat receiver. Using the annular compound parabolic concentrator can reduce the surface area of the heat receiver, increase the concentration ratio of the system, then raise the system's power generation efficiency. Simulation and experimental results show that, the annular compound parabolic concentrator increases the system's concentration ratio by about 94.4% and extend the system's daily working hours, especially in cloudy days, so as to improve the generating efficiency by about 3 percentage points.