Under the situation of ideal channel characteristics, the spacetime- polarization processing (STPAP) technique can mitigate interferences effectively in the space domain, time domain and polarization domain. However, channel mismatch is inevitable in practice, which will cause the amplitude and phase error. In the current work, the effect of channel mismatch on the performance of the STPAP is investigated. Firstly, the STPAP architecture and the channel mismatch model are established. Then, the received signal model under the situation of no channel mismatch is put forward. After that, a novel channel mismatch model for GNSS signals is proposed, based on which the received signal model under the situation of channel mismatch is presented. Furthermore, the effect of channel mismatch on the performance of the STPAP is analyzed in theory. Finally, simulation results indicate that channel mismatch can greatly degrade the performance of the STPAP.