An actuator is provided, including a fixed assembly and a movable assembly. The fixed assembly includes a coil module, a base, a first screwing member, and a linear rail. The first screwing member passes through the base and the linear rail, and the linear rail is positioned on the base. The movable assembly includes a U-shaped back board having an inner space, a first magnetic module, a second magnetic module aligned with the first magnetic module, and a sliding block. The first and second magnetic modules are disposed on the U-shaped back board and accommodated in the inner space. The coil module is disposed between the first magnetic module and the second magnetic module. The sliding block is positioned on the U-shaped back board in the inner space, and slidably connected to the linear rail.