Control of the air-fuel ratio (AFR) in gasoline engines is of imminent importance when aiming at minimizing calibration effort and meeting performance requirements. People have higher demands on the gasoline engine, which has less exhaust emission, better economic efficiency and favorable engine power performance. In order to keep the air-fuel ratio close to the stoichiometric value under transient conditions, AFR precise control was achieved by employing triple-step method which was easily to be implemented in engineering. The structure of the designed controller consisted of three parts: steady-state control, feed-forward control concerning the reference variations and error feedback control. When the desired AFR was a constant, the steady-state control would play a dominant role. And the feed-forward control would react immediately when the desired AFR was changed on account of the torque requirements changed. The feedback control would amendment AFR measured by the exhaust gas oxygen (EGO) sensor which can enhance the close-loop performance and rearranged into a state-dependent PI. A straightforward design process was provided, and the structure of the designed non-linear controller was easily achieved, which was comparable to those widely used in current automotive control. Furthermore, taking the implement delay of the injection into consideration, it can be compensated by feed-forward control based on predicting the intake manifold pressure. Finally, the simulation results in the environment of en-DYNA with a reasonable common fourcylinder engine model showed the efficiency of the proposed method. And the predicted intake manifold pressure was visibly advanced to that without prediction one in the simulation result.