In the photovoltaic power generation process, the maximum power point of photovoltaic cell requires tracking control, which aims to improve the battery utilization and power generation efficiency. The principle of the fundamental disturbance observer method is analyzed, as well as its advantages and disadvantages. And then, an improved algorithm is proposed by adding a re-judgment of the climbing slope, which is considered to be the criterion for junction point of variable step size. Combining with the incremental conductance algorithm, the maximum power point of photovoltaic cell is tracked and controlled by using the variable step and weak oscillation method. Finally, the simulation results show the correctness of this algorithm. The results show that the improved tracking algorithm is more stable, and eliminates the oscillation and improves the accuracy. The dynamic response is better.