The phenomena of self-mode locking in Q-switched Nd:YAG laser used for pumping Tisapphire will result in instability of laser pulse peak power output and mangle the optical devices. In order to eliminate this phenomena availably, the reason caused for self-mode locking is investigated, and Fabry-Perot (F-P) etalon with different parameters is employed. By using one piece of 20 mm-thick F-P etalon with a reflectivity of 60%, which can not only reduce inserting loss but also avoid the damage of F-P etalon′s coatings, a smooth pulse waveform in pulse width of 70 ns and pulse energy of 600 mJ at 1064 nm is realized, the reason for it may be as follows: though there are probably five longitudinal modes in a full width at half maximum (FWHM) of F-P etalon theoretically, only one longitudinal mode may be able to oscillate considering the loss; though there are several transmission peaks in the gain bandwidth of Nd:YAG, incontiguous longitudinal modes can not meet the condition of mode locking.