The stable mode-lockingoperation of a pulsed Nd:YAG laser was achieved by inserting a LiF:F-2 crystal into the critical stableplane-concave resonator. This paper analyzed the action of nonlinear absorption of LiF:F-2 inmode-locking progress, and calculated the changes of stable range of the resonator and thelaser mode sizes, due to self-focus of Nd:YAG. The theoretical and experimental results showed that the Kerrlens effect was the key factor of mode-locking operation for the Nd:YAG laser.