The invention discloses a photovoltaic power generation system online IV curve test device and a test method. The online IV curve test device comprises a temperature acquisition system, a solar irradiance gauge, a multi-channel photovoltaic assembly IV tester, an industrial control computer, a switch, a host computer and a switching case. According to the online IV curve test device, a relay with relatively low cost and having no arc discharge is employed to switch photovoltaic assemblies, IV curves of the assemblies are respectively tested through rapid scanning, accurate volt-ampere characteristics of the photovoltaic assemblies are acquired, and the online IV curve test device satisfies online IV curve tests on all channel assembly strings through cascade networking at present. Through the method, after switching one assembly at each time, an output voltage of the residual assembly string is guaranteed to be greater than the lowest input work voltage of an inverter, and a cell input power is further guaranteed to be greater than a normal start power of the inverter. The whole test period of rapid assembly switching, rapid IV curve scanning test and rapid switching recovery is controlled to be quite short, the online IV curve test function is realized.