Internal parallel moving gears transmission is a new type of planetary transmission with small tooth number difference. The dynamic model of the system is established by using lumped mass method. In the modeling process, the factors including the elasticity of all eccentric shafts, the elasticity of planetary bearing, the elasticity of supporting bearing on output shaft and the elasticity of meshing gears are considered, and the coordinate relation of the system is derived. The practical example and computer simulation was carried out and the dynamic characteristic is obtained. The results show that the gear transmission becomes steadier when simultaneously inputting the power from the three symmetrical shaft and utilizing multi-phase parallel mechanism, and the load of supporting bearing can be greatly reduced. This paper offers theoretical basis on correctly designing internal parallel moving gears transmission, reducing moving load and extending the service life.