Hydraulic drive mixers are widely employed to process the cohesive mixture, especially the dusty and energetic mixture. In this study, the modeling method of the AC frequency control speed regulating system of a planetary mixer was presented. The speed regulating system consists of links of frequency converter(FC), asynchronous motor(AM), and hydraulic circuit(HC). The stability analysis was conducted by applying specification parameters of the FC, AM and HC. Result shows that the speed regulating system is stable by Routh criterion with amplitude margin 21.4db and phase margin 17.9degree.