Higher requirements have been put forward on accuracy and real-time of frequency detection, because renewable energy access to the grid in large-scale. The existing frequency detection method must rely on the phase-locked loop (PLL), considering the problem of PLL in the background of power electronic, a new frequency detection method is proposed in this paper based on synchronous phase open-loop detection of power grid. In this method, the difference-frequency phase is extracted from the phase information obtained by the open-loop phase detection and difference-frequency is obtained by adjusting difference-frequency phase. The experimental results show that the proposed method can effectively detect the real-time frequency of the power grid, which has high precision and strong anti-interference ability.