Though with the advantages in its control performance, the traditional variable step length Inc algorithm has its own disadvantage as well: a large amount of computation which affects the tracking speed and a large stable step length which fails to keep the system stable. In view of this, based on Matlab/Simulink platform, a control model of photovoltaic maximum power point tracking has thus been established. By using S-function programming in Simulink, the simulation of a new variable step size Inc algorithm can be realized, thus helping to obtain relevant conclusions. The simulation results show that the improved variable step length Inc algorithm, which is based on S-function, is characterized with a faster tracking speed, accurate and stable in its tracking at the maximum power point.