A linear actuator includes a linear module including a base, a screw rod mounted in the base and a transmission carriage threaded onto the screw rod, and a support module including two support carriages movably mounted on the screw rod at two opposite sides relative to the transmission carriage, a protective cover disposed at one side of the base and connected to the two support carriages, two pulley blocks mounted at the two opposite ends of the protective cover and a cable wound round the pulleys of the pulley blocks and connected to the transmission carriage and the base by a respective cable connection plate. Thus, the transmission carriage utilizes the cable to carry one pulley block and then the protective cover, causing the protective cover to carry the two support carriages and the other pulley block, achieving the effect of supporting the screw rod.