A linear actuator assembling method includes a semi-finished product assembling step and a threaded shaft assembling step. The semi-finished product assembling step is to form a semi-finished product, and the semi-finished product includes a case, a main gear, and a main gear bearing. The main gear and the main gear bearing are surrounded and restricted by the case, and the main gear bearing is sleeved on the main gear. In the threaded shaft assembling step, a threaded shaft is reversely assembled, and one end of the threaded shaft is inserted into the main gear to allow the end of the threaded shaft to be restricted by the main gear.