To enhance the knowledge of non-uniform heat flux distribution of receiver in a solar power tower plant, an optical simulation software, SolTrace, was used to model the Gemasolar plant system geometrically and optically, and the wall heat flux distribution of the receiver was obtained. According to the convergence judgment, 5 000 000 rays were used for follow-up study. The accuracy of the model was verified by comparison with results from the litearture. Under the standard condition at the design point on Summer Solstice, the incident radiation of sunlight was adopted. From the simulation results, it is found that the variation of the solar radiation heat flux received on the wall of different receiver tubes is related to the annular distribution of the heliostat field. For a single absorber, the wall heat flux distribution is non-uniform, and the comparison with the Gaussian approximation distribution proves that this distribution is quite different from the design condition. Then, the heat flux distribution of the receiver at noon of Winter Solstice was obtained by using the model. Overall, it is concluded that SolTrace simulation is more practical than the simplified Gaussian approximation distribution for the treatment of wall heat flux distribution of external cylindrical absorber.