Abstract: In recent years, there are many explosion accidents of portable electronic devices, which are mainly due to the electrolyte leakage of the separator/electrolyte system of traditional lithium-ion batteries and severe heat shrinkage of the separator resulting in short-circuiting between positive and negative electrodes. The appearance of polymer electrolytes has greatly improved the safety of traditional lithium-ion batteries. Polymer electrolytes mainly include all-solid polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs). For all-solid polymer electrolytes, it still needs a long way for their industrial application because they have low ionic conductivity (10^-6-10^-8 S/cm) at ambient temperature, which does not meet the basic requirements for practical use. GPEs are a hybrid system formed by trapping liquid electrolyte in a polymer matrix, combining the advantages of high safety of solid electrolyte and high ion conductivity of liquid electrolyte. Among them, cross-linked GPEs have received extensive attention. The cross-linking networks prevent the leakage of electrolyte and improve the mechanical properties and dimensional stability of polymer matrix, so the battery safety performance can be greatly improved. Cross-linked GPEs prepared by thermal initiation, photoinitiation, radiation initiation, epoxy ring-opening addition, and sol-gel methods are reviewed according to their initiating and preparing methods, meanwhile their mechanical and electrochemical properties are also introduced.