A diode pumped injection seeded single-longitudinal-mode(SLM)Nd:YAG laser is achieved by using the resonancedetection technique in Q-switching operation.The pulsed oscillator laser uses a folded cavity to achieve compact construction.This system operates at 100Hz and provides over 20mJ/pulse of single-frequency 1064nm output.The M2 values of horizontal and vertical axes are 1.58 and 1.41,respectively.The probability of putting out single-longitudinal-mode pulses is 100%.The 355 nm laser output produced by frequency tripling has a linewidth less than 200 MHz.The laser can run over eight hours continually without mode hopping.