According to the characteristics of small bore spinning missile, the design scheme of integrated navigation algorithm based on geomagnetism assisted MEMS inertial measurement unit (IMU) is proposed. The error mathematical models of MEMS inertial devices and geomagnetic sensors are given, and the calculation principle of 3-axis geomagnetic attitude determination is briefly described. The MEMS inertial navigation simulation and MEMS IMU/geomagnetic integrated navigation simulation are carried out on the designed flight trajectory. The results show that the attitude angle error and position error of MEMS inertial navigation system with geomagnetism assistance are significantly reduced, which can meet the requirements of the spinning missile.