A kind of thermal error modeling method of a motorized spindle based on natural exponential was proposed. Through the analysis of the structure and temperature field of the motorized spindle, the geometric model and thermal boundary condition which were used to analyze the thermal characteristic of the motorized spindle were achieved. Then the thermal characteristic Finite Element Analysis (FEA) was proceeded to get the steady thermal field and the steady deformation field. The thermal error of motorized spindle under random rotating speed could be described by a nature exponential model. However, it was necessary to get two parameters: the thermal balance time and the steady thermal error under random rotating speed, so the calculating method of the two parameters was provided here. An experiment was conducted on a machining center to measure the motorized spindle’s thermal error under random rotating speed. The result shows that of the exponential model is perfectly good in both the accuracy and the robustness.