A brake device for a linear actuator is provided having a torque transmission mechanism for forward transfer of driving torque and a spring friction mechanism for blocking backward transfer of load torque. The mechanism provides an input shaft and a first coupled claw connected to the input shaft that rotates synchronously therewith, a driving torque output shaft and a second coupled claw connected to the output shaft and capable rotating synchronously therewith. The first coupled claw and second coupled claw are matched and engaged together, and used to transfer driving torque in a forward direction from the input shaft to the output shaft. The spring friction mechanism provides a brake housing and a friction coil spring disposed elastically in a cylindrical inner hole of the brake housing. The friction coil spring formed with end parts at two axial ends for hooking onto the first and second coupled claws.