The commercialization of lithium metal batteries (LMBs) highly relies on developing high-performance solid electrolytes, which should possess high Li+ conductivity comparable to liquid electrolytes, efficient suppression of dendrite growth, nonflammability, intimate electrode/electrolyte contact, and compatibility with the state-of-art lithium batteries industrial fabrication. Herein, we develop an in situ crosslinking method via cationic ring-opening polymerization (CROP) using a four-armed cross-linker and fabricate a crosslinked gel polymer elec-trolyte (c-GPE), where the more densely and efficiently three-dimensional (3D) crosslinked polymer network renders the c-GPE high solvent uptake and improved oxidative stability. Moreover, the strong interaction be-tween the crosslinked polymer network and the solvent is proved to reduce the desolvation energy barrier of Li+, which facilitates homogeneous Li+ deposition. Thus, the Li||LiFePO4 battery with this in situ fabricated c-GPE demonstrates one of the longest lifespans among polymer electrolyte-based batteries at high rate (2C) so far (2000 cycles with 78% retention). Furthermore, when this in situ 3D crosslinked gel polymer electrolyte is applied together with high voltage cathode material Ni0.6Mn0.2Co0.2O2, the Li|c-GPE|Ni0.6Mn0.2Co0.2O2 battery could deliver the best cycling performance with high nonflammability among CROP-based batteries so far (300 cycles with 80% retention). These results demonstrate that by the dedicated design of in situ polymerization, a high-performance c-GPE could be achieved to meet the requirements for practical, safe, and high-energy-density LMBs.