In order to improve the conversion efficiency of grid-connected PV system, a new method to realize maximum power point tracking - recursive Calculation were proposed. This method is the direct calculation of the maximum power point, and its hardware uses a DC/DC boosted circuit to regulate PWM duty cycle. Only through a sampling, the voltage of the maximum power point and the duty cycle value can be calculated directly. By adjusting the value of duty cycle, the values of voltage and current can be got recursively, and then determine whether the derivative that power divide voltage equals to zero. If zero, it indicates the maximum power point has been reached. By DSP2812 this method has been verified that it shows rapid computing speed, high accuracy, and high stability of the system.