A pulsed single-longitude-mode laser which uses a Fabry-Perot (F-P) etalon to select mode has advantages of simple compact structure and tunable wavelength. However, the output energy of the laser is sensitive to the change of cavity length, and the long-term stability is poor. To solve these problem, the main factors affecting the stability of output energy are analyzed based on the principle of single-longitude-mode laser with F-P etalon. The relationship among frequency, resonant cavity length and energy of output laser is obtained. An energy stability control method of dynamic feedback control cavity length is proposed. A high-speed digital circuit energy stabilization system based on field programmable gate array (FPGA) is designed. The length of resonant cavity is controlled periodically with the proposed method, and the intensity of the nanosecond pulsed laser signal is detected by photodiode. The frequency variation of longitudinal caused by the cavity length modulation directly affects the intensity of the output laser signal. The information of laser intensity change is analyzed, and the cavity length detuning is calculated. Based on this, the cavity length compensation is realized and a closed-loop control system is formed. Finally, the long-term stable output of the single-longitude-mode laser is achieved. Experimental results show that energy stability of the nanosecond pulsed single-longitude-mode laser is significantly improved with the proposed control system. The energy instability of continuous working for 3 h is 1.3%.