Islanding detection is an important guarantee for the safe operation of photovoltaic grid connected system. At present, there are a lot of improvement researches on slidemode frequency(SMS) and OFP/UFP islanding detection method（IDM）, but the disadvantage of large nondetection zone (NDZ) of OFP/UFP is not changed when the matching degree of load consumption power and inverter output power is high, the disadvantage of large influence to power quality of SMS is not changed. A mixed islanding detection method based on SMS and OFP/UFP is proposed. The NDZ of OFP/UFP and the influence to power quality of SMS is decreased. The malfunction of the system which is caused by the abrupt change of the system was avoided. It can switch different IDM algorithms when the load is changed. Islanding detection experiment is carried out in MATLAB＼Simulink and photovoltaic microgrid experiment system. The experimental result verifies the correctness of theoretical analysis.