Solid-state polymer electrolyte lithium metal batteries (SPE-LIBs) with high energy density and safety have been widely considered as the next generation of lithium batteries. In recent years, the ion transport mechanism of SPE has been extensively studied, resulting in a significant increase in its ionic conductivity and lithium transference number. However, SPE has poor cycling stability when used with high-voltage cathodes, limiting its further development. Therefore, it is important to understand the mechanism of battery performance degradation in high-voltage systems. In this work, first, the SPE was prepared by photopolymerization using 1-butyl-3-vinylimidazolium bis(trifluoromethylsulfonyl) imide, vinylethylene carbonate and poly(ethylene glycol) diacrylate. The LiFePO₄(LFP)||SPE||Li battery was prepared by the in-situ polymerization method. This battery has an initial discharge specific capacity of 159.6 mAh•g^-1 at room temperature, and the capacity retention rate reaches 95% after 145 cycles, indicating that the cell has a low interface impedance and the electrolyte has a high ionic conductivity. Then, in-situ Scanning Kelvin Probe Force Microscopy was used to characterize the interfacial potential of the cross-sections of batteries with two different cathodes, which were prepared by argon ion beam polishing, during charging. Compared with the LFP cathode, the LiNi_0.6Co_0.2Mn_0.2O₂ (NCM) cathode with higher electrochemical reaction potential has a greater potential difference between the cathode and the electrolyte after charging. As a result, it indicates that the energy level of high-voltage cathode materials changes greatly during the charging process, which makes the electrolyte materials more susceptible to losing electrons preferentially and cause degradation side reactions. Combined with electrochemical impedance spectroscopy and laser confocal Raman spectroscopy to characterize the interfacial structure of the SPE-LIB. It found that side reactions would destroy the structure of the cathode electrolyte interphase, resulting in a significant increase in the interfacial impedance and the attenuation of the battery capacity. The electrolyte degradation mechanism of SPE-LIBs under high-voltage systems was revealed through in-situ characterization, which provided guidance for improving the cycling stability with high-voltage cathodes.