A safety mechanism for an electric linear actuator is disclosed. The linear actuator has a guide screw and a telescopic tube. The safety mechanism 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 to form an integer 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.