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