Abstract: Single-longitudinal-mode (SLM) lasers, characterized by their superior output laser beam quality, narrow linewidth, stable frequency, compact size, and efficient heat dissipation, find extensive applications in coherent optical communication, lidar, nonlinear optics, gravitational wave detection, and high-precision spectral measurement. In this study, we employed a short cavity scheme in conjunction with Fabry-Perot etalon mode selection, and carried out a theoretical analysis of the longitudinal mode within the cavity. Our design involves a laser diode single end-pumped acousto-optic Q-switched Nd: YVO₄ laser with a cavity length of 90 mm, into which a 5 mm Fabry-Perot etalon with a reflectivity of 90% is inserted. This setup yielded a SLM laser output with an adjustable repetition frequency of 1 Hz~10 Hz, a pulse duration of 10 ns, an energy of 133 μJ, and an energy instability of 3.39%. The output SLM laser was further amplified by two stages, resulting in a single pulse energy of 40 mJ.