An electric linear actuator has a guide screw and a telescopic tube. A safety mechanism for the linear actuator includes a nut, a nut sheath, a sleeve, and a torsion spring. The nut connects the guide screw. The nut sheath is fixed on one side of the nut and is provided with a plurality of bumps. One end of the sleeve is put around the nut sheath and the other end thereof is connected to the telescopic tube. The sleeve is provided with a plurality of rabbets corresponding to the bumps. The torsion spring is disposed around the nut sheath and the sleeve. When the guide screw is rotated, the nut sheath is engaged with the sleeve by embedding the bumps into the rabbets. When the sleeve stops, the bumps and the rabbets depart from engagement to make the nut sheath and the nut freely rotate against the sleeve.