Taking Cr4+:YAGs as saturable absorbers and reflective volume Bragg gratings (RVBG) as output mirror, passively Q-switched operation of flash-lamp pumped Nd:YAG single-longitudinal-mode (SLM) laser was demonstrated. Single-frequency operation is accomplished for all output powers by twisted mode cavity and RVBG. This method can bring higher energy and stability. The maximum linewidth of SLM laser is about 78 MHz. The curve of energy and pulse width have been obtained under conditions of several Cr4+:YAGs with different initial transmissivity. The maximum SLM laser output energy is 20.8 mJ, minimum pulse duration is 13.2 ns and peak power is 1.18 MW.