A mathematical model for three-dimensional simulation of unidirectional solidification process and microstructure evolution of Ni-based superalloy investment castings was developed based on CA-FD method. The modified ray tracing method was used to solve the complicated heat radiation transfer among the multiple blades and outer space during withdrawal process. Various withdrawal rates were used. During one process high withdrawal rate was used first before the platform approached the baffle. Then the low withdrawal rate was used to reduce the temperature difference of the platform in horizontal section and avoid the defects formed in the corner of the platform. The experimental cooling curves of different positions in the blades and microstructure were compared with the simulation results. Both the results showed that the various withdrawal rates process was effective to reduce the temperature difference of the platform and avoid the formation of stray grains. This process could be helpful to increase the productivity.