Polymer flooding is an important chemical EOR technology for many Chinese reservoirs. Polymer thermal degradation is an important factor affecting the polymer flooding. It is necessary to consider the polymer thermal degradation when polymer flooding project is designed using reservoir numerical simulation. Currently，numerical simulation of polymer flooding considering polymer thermal degradation is not yet perfect. Based on viscosity attenuation model that can accurately reflect the polymer thermal degradation，method of streamlines updating at each step and mapping between grid and streamline was applied to solve the degradation time，and the numerical simulation of polymer flooding considering polymer thermal degradation can be realized. The method has clear physical meaning，strong applicability and accurate calculation results，reflecting the thermal degradation process of flowing polymer underground accurately. Three models were studied using the new method. The results show that the time of polymer degradation is less than that of propagation in the stratum during polymer flooding process，and equal when the process is stable. Thermal degradation has no effect on polymer concentration，but decreases the viscosity significantly. As a result，the effective time of polymer flooding is prolonged，water cut drop funnel is minified and the effect becomes poor. It is necessary to consider the polymer thermal degradation when designing polymer flooding project.