The three-phase four-leg grid-connected converter can be obtained by adding a bridge into the traditional three-phase three-leg grid-connected converter. The cycle by cycle switch average model (CCA) based on three-phase four-leg grid-connected converter is presented in the paper, and the model shows the converter can de decoupled to three single phase full bridge converters. The SPWM (Sinusoidal Pulse Width Modulation) and SAPWM (Saddle Pulse Width Modulation) control strategy of converter are analyzed respectively. Compared with the SPWM control strategy, the SAPWM control strategy is benefit to lower output DC voltage and reduce the voltage level of switches. The three-phase four-leg grid-connected converter has rigid adaptability to the power system whether it is three-phase balanced or unbalanced. Experimental results based on a 5-kW three-phase four-leg converter prototype verified that the converter achieves excellent performance.