A dynamic model of light-weight spur gear transmission system (SGTS) considering the elasticity of shaft and gear body is established to study the influences of the elasticity of the gear body on the dynamic characteristics. The spur gear is simplified as elastic ring and support springs stemming from the elasticity of the gear web. The shaft and ring are modelled based on Timoshenko beam theory. The kinetic energy of the elastic shaft-ring gear rotor is obtained based on multibody dynamic theory. A time-varying meshing model is proposed considering the movements of the meshing points along the base circles of gears. The differential equations of the SGTS are formulated by differential quadrature finite element method (DQFEM). The present model is validated by compared to lumped-mass model (LMM) by setting the elasticity of the gear body to be infinitely small. The influences of the elasticity of the gear web are studied. The results show that the elasticity of the gear web can reduce the intensity of the vibration.