In photoacoustic imaging (PAI), nanosecond laser with controllable pulse width (PW) and pulse number (PN) benefits different imaging applications, which require variable PA frequencies or nonlinear excitations. Here, we report a compact (1-mm-long cavity) gain-switched laser that directly generates tunable PW (18 - 110 ns), and PN (1 - 4) with pulse intervals ranging from 236 to 553 ns, as well as adjustable repetition rate (5 - 35 kHz). The controllability utilizes the relaxation oscillation effect in the laser oscillator by manipulating the pump and cavity conditions. We showcase the effectiveness of tunable PW excitation in photoacoustic microscopy and the PA enhancement of 33.9% under multi-pulse excitation. The promising performance suggests that this kind of laser can be flexibly applied in functional and nonlinear PAI systems.