Abstract: In this paper, mainly discuss the synthesis method of combined control systems (systems both contain open loop and closed loop control circuits). Suggest a simple method to calculate the system integral-square error by its logarithemic frequency characteristics. This method can easily be applied to synthesize the feedforward networks. When the output of the actuating element is subjected to some integral limit conditions, or the discrete circuit is used as feedforward unit, the synthesis method of feedforward network is also discussed.