A linear actuator includes a main body and a gear box structure. The main body includes a housing, a motor, a composite worm gear assembly and a threaded rod. The housing includes an inner space. The composite worm gear assembly is disposed in the inner space and driven by the motor. The threaded rod is connected to the composite worm gear assembly. The gear box structure is detachably connected to the main body, and includes a lower cover, an upper cover and a controlling unit. The lower cover is detachably embedded in the housing, and includes a side notch and a lower notch. The upper cover is slidably and correspondingly covered with the lower cover to form an accommodation space. The controlling unit is disposed in the inner space. The side notch is corresponding to the composite worm gear assembly. The lower notch is corresponding to the motor.