A model based on propagation rate equations is built up for analyzing the multicore transverse mode gain distribution in an 18-core photonic crystal fiber (PCF) laser. The two kinds of feedback cavities are used for the fiber laser, which are the buttcontact mirror and the Talbot cavity. According to the model, the transverse mode competitions in different feedback cavities are simulated numerically. The results show that the Talbot cavity can improve in-phase supermode gain, while suppress other supermodes.