The present invention relates to a method for injecting an electrolyte into a battery cell where an electrode assembly is built in an accommodation unit of a battery case and an electrolyte injection port is not sealed in a process for manufacturing the battery cell. The method comprises a disposition process (a) of disposing the non-sealed battery cell inside an injection chamber connected to an injection tank where an electrolyte is stored; a first decompression process (b) of performing vacuum suction until pressure in the injection chamber reaches a first vacuum state; a second decompression process (c) of increasing the pressure until the pressure in the injection chamber reaches a second vacuum state; an injection process (d) of injecting electrolyte in the injection tank through the injection port of the non-sealed battery cell while maintaining the second vacuum state; and a vent process (e) of releasing a vacuum state of the injection chamber in order to make the pressure in the injection chamber get equilibrium with atmospheric pressure. According to the present invention, two-stage decompression processes are applied to shorten the time for injection of an electrolyte and enhance impregnation performance of an electrolyte.