Island detection is an important function of photovoltaic gridconnected power generation system. In order to reduce the dead zone detection effect of the island detection device and its impact on power quality, this paper proposes an improved SMS algorithm based on the research of traditional sliding mode frequency offset (SMS). Its working principle of the algorithm and detection dead zone are analyzed. Then it is applied in the digital signal processing (DSP) environment. The simulation study is performed in the case of a single inverter and multiple inverters in parallel operation. The simulation results show that the method can not only ensure a short detection time, but also reduce the impact on the power quality of the power grid, thus verifying the effectiveness and practicability of the method.