A liquid injection device includes a nozzle (10), a moving element (20) and a control valve (30). The nozzle (10) has a channel (11), a bypass passage (12) and a suck back passage (13). The channel (11) penetrates through the nozzle (10) for injecting a working fluid. The channel (11) has a liquid outlet (111). The bypass passage (12) has a first opening (121), a switch-on position (122) and a switch-off position (123). The suck back passage (13) has a second opening (131) between the first opening (121) and the liquid outlet (111). The first opening (121) and the second opening (131) communicate with the channel (11). The moving element (20) is disposed in the bypass passage (12). The control valve (30) is disposed on the nozzle (10) and controls the moving element (20) to switch between the switch-on position (122) and the switch-off position (123) to open or close the channel (11). A suck back pump (B) sucks the working fluid remaining between the first opening (121) and the liquid outlet (111) when the moving element (20) is located at the switch-off position (123).