Implements a dynamic frequency measure method based on software phase-locked loop(SPLL),and analyses the resolution in detail.The method uses the correspondence between the TMS320F2812 timer counts and PI regulator output error of the angular frequency, to realize timer periodic interrupt and add a fixed angle once a interrupt. The frequency measurement can be realized by altering the timer counts of the periodic interrupt. There is no need for extra hardware integrated circuit in the novel method and the measuring speed is fast. The experiments show that the method is feasible.