Based on dual orthogonal digital filters and weighted smoothing phase-difference method， an algorithm for tracking instantaneous frequency of power grid is proposed， which improves the dual orthogonal digital filter used currently. Detailed simulations on several typical models of power signals have been carried out to verify the algorithm’s dynamic characteristics. Simulation results show that the frequency tracking errors are 0.000 1 Hz and 0.005 Hz respectively for constant-frequency and fluctuating-frequency power signals. The impact factors of tracking error and time delay are analyzed accordingly. The results show that， the algorithm is not subjected to the synchronized sampling criteria and can track the instantaneous frequency of different kinds of power signals accurately and instantly.