A stable and wavelength-locked Q-switched narrow-linewidth Er:YAG laser with compact cavity structure, utilizing a volume Bragg grating (VBG) as a wavelength selector and a pump input mirror simultaneously, is reported. It yields high energy nanosecond pulse with pulse duration of 185 ns and pulse energy of 1.36 mJ at 1 kHz pulse repetition frequency for incident pump power of 21.6 W. The central wavelength of the Er:YAG laser is locked at 1645.3 nm with a spectral 3-dB linewidth of less than 0.08 nm, which coincides to the methane (CH4) absorption-line. The output has near diffraction-limited beam quality with M2 parameter of 1.08. Our work may provide an inroad for developing more miniaturized space-based integrated path differential absorption (IDPA) lidar transmitter.