To shorten the design schedule of a direct drive servo system and to ensure its performance to meet the indicator demand of the system design, a mechatronic modeling method for the direct drive servo system was proposed. By taking a typical uniaxial direct drive system for an example, the implementation process of the method was described. The dynamics model of direct drive system was established, and the mechatronic co-simulation model based on Matlab and Recurdyn was also build. The time-frequency domain characteristics of the mechatronic co-simulation model was analyzed and simulated in different conditions, then their simulation and experiment results were compared. Comparison results show that open-loop time-frequency domain simulation and an pratical experiment results are basically consistent, and its Amplitude-frequency Characteristic Matching Degree (ACMD) is greater than 83%, Phase-frequency Characteristic Matching Degree (PCMD) greater than 73%, and Time-domain Response Matching Degree (TRMD) greater than 89%. It verifies the correctness and effevtiveness of the mechatronic co-designing method. The method is suitable for predicting the system performance in the mechatronic design process, which improves design efficiency.