The invention provides an auxiliary gas-phase cross-linking method of electron beams. The method comprises the following steps: (1) loading a polymer fiber sample in an under-beam apparatus, vacuumizing, replacing high-purity nitrogen, and repeating twice; (2) respectively introducing active gas and high-purity nitrogen, wherein a ratio of the high-purity nitrogen flow to the active gas flow is (1 to 1) to (1 to 5); (3) rotating the polymer fibers in the under-beam apparatus at a rotation speed of 5 to 20 rad/min; and (4) starting an electron accelerator, wherein the beam density is 0.5 to 2 kGy/s, and stopping when the accumulative radiation amount is more than 8MGy. The cross-linking degree of the polymer fibers can be judged by testing a gel content of a radiation sample; and the ordinary gel content is greater than 90 percent, thus achieving the cross-linking target.