A kind of digital photovoltaic pumping system was designed, which is simple in structure, based on 16 bits single-chip microprocessor. The system uses the method of frequency differential approximation to realize the true maximun power point tracking(TMPPT) function of solar cells. The result proves that the system runs very reliably, and the efficiency is higher than that of the traditional contant voltage tracking(CVT) method. In addition, the system has perfect protection functions, and it can run without operators.