In the traditional perturbation and observation method of maximum power point tracking (MPPT),there exist the MPPT accuracy low and oscillation problem. On the basis of the traditional MPPT algorithms. In this paper.An improved algorithms is proposed, which combined a variable-step perturbation and observation method with Newton interpolation method. The improved algorithms is simulated and applied in the three-phase grid connected power system. The results show that the improved MPPT algorithm not only can quickly track the changes of light intensity, but also can effectively reduce the oscillation around the maximum power point, and improve the efficiency of photovoltaic cells.