In this work the degradation behavior of sulfonated polyimide-hydrogen type (SPI-H) membrane for vanadium redox flow battery (VRFB) usage was investigated through in-situ and ex-situ measurements. Morphological, mechanical and physico-chemical properties, together with structure information of the SPI-H membrane or its oligomer before and after degradation were characterized. Both optical photographs and SEM images show more serious destruction of surface morphology of SPI-H membrane during in-situ degradation than ex-situ one. Decrease in mechanical property and viscosity average molecular weight of SPI-H membrane after degradation in different immersion solutions verifies that both H+ and V(V) play important roles in accelerating the decomposition. FTIR results demonstrate that -SO3H groups of SPI-H membrane in VRFB aren't destructed. H-1-NMR spectra verify the scission of polymer chain and the hydrolysis of imide ring of SPI-H membrane after degradation. XPS spectra evidence the oxidation of-NH2 groups of one hydrolysis product of SPI-H. According to all the experimental results, the 2-step degradation mechanism including hydrolysis and oxidation of SPI-H membrane in VRFB is proposed.