Abstract: To solve the problem of the error of frequency measurement algorithm with timevarying frequency in mine power supply system, an improved software zerocrossing algorithm is proposed. The relationship among the phase angle, the time and the frequency of the voltage of the system is established. The relation formula between the realtime frequency and the period of zerocrossing point is derived when rate of frequency variation is not zero according to the relationship between two adjacent with the direction of the zerocrossing phase difference of 2π. The exact calculation formula of the period is obtained by using the Newton three interpolation polynomial, the Newton iteration method and the Qin Jiushao algorithm. Hence, the frequency and rate of frequency variation can be obtained. A variety of situations ,including signals without harmonics, signals with harmonics and random noise, as well as the frequency rapid decline process in system failure, are simulated using MATLAB, The simulation results show that the algorithm of this paper can accurately measure the realtime frequency of the system and rate of frequency variation in above situations, and this method is superior to conventional zerocrossing algorithm in measurement accuracy, instantaneity and computation. This algorithm is suitable for application of protection, control and monitoring devices and automation equipment in mine intelligent power supply system.