Global Navigation Satellite System (GNSS) signal simulator is one of the most important instruments for testing the function and performance of GNSS terminal products. It consists of a Mathematical Simulation System (MSS) and a Radio Frequency Signal Generator (RFSG). The MSS is responsible for generating the navigation message and observation data and then sending them to the RFSG in real time. The simulation accuracy of each part in the MSS determines that of the signal generated by the RFSG. In this paper, the function requirements of each module in MSS are analyzed firstly. Then the general framework of MSS is presented. At last, MSS developed in this paper is demonstrated and tested partly, the test results are compared with that of SimGEN software and STK. Taken as a whole, the MSS has complete functions and good performance. The MSS will hopefully apply to GNSS signal simulator.