In order to study the influence of the spindle span design on the dynamic mechanical properties， the best span of a certain type of electric spindle was calculated based on relevant empirical formula, and the natural frequency and static stiffness under different spans were simulated. The stiffness of spindle bearing was verified through the static stiffness loading experiment of spindle to avoid the influence on the simulation model. Based on the boundary condition set for simulation model, a modal analysis simulation model and a static analysis simulation model were established;by changing the span of the spindle, the first order natural frequency and radial static stiffness of the spindle were analyzed, and the influence degree of the design span of the motorized spindle on mechanical properties was analyzed and summarized; based on the theoretical calculation value of the best span, the general principle of motorized spindle span design was put forward.