The invention provides an isolated island operation detection method of photovoltaic grid-connected generation system which includes: a control device detects voltage frequency of grid-connected point, judges voltage frequency of grid-connected point whether reaching frequency protection threshold, if yes, a start anti-isolated island protection signal is outputted; if not, frequency signal of grid-connected current instruction is calculated by using double positive feedback active frequency deflection method, and the frequency signal is outputted; the double positive feedback active frequencydeflection method is that obtained grid-connected point voltage frequency deflection relative to average frequency is used as first positive feedback, and change trend of frequency deflection is usedas second positive feedback, feedback magnification factor of first positive feedback is adjusted. The method realizes isolated island detection rapidly, simulation and experimental result, isolatedisland detection time is shorten in 0.10s, 0.13s, grid-connected current harmonic distortion THD are 2.06%, 2.97% respectively, and has advantage over traditional method in isolated island detection speed and reducing interference to power network.