A safety linear actuator assembly includes an actuator including a hollow holder frame, a gear set mounted in hollow holder frame, a driver for rotating gear set and a transmission mechanism that includes a lead screw having a stem located at one end and coupled to gear set and a pin mounted in a pin hole at stem, two bearing holders mounted on stem at two sides of the pin and two thrust bearings mounted on the stem and respectively accommodated in the bearing holders, and a retaining ring mounted in a locating groove at the stem. If the stem breaks accidentally due to that the lead screw bears excessive torsion and impact, the retaining ring will be forced against the respective bearing holders and the hollow holder frame, stopping the lead screw from escaping out of the hollow holder frame and assuring a high level of operational safety.