Optical resonant reflector (RR), as a coupled-output mirror of Nd∶YAG oscillator, has very strong ability for selecting longitudinal mode. Three-plan RR has more regular structure of modulated reflecting-spectrum. For outputting stable single longitudinal mode, it is optimized for oscillator parametric designing in theory and it is suggested that the oscillator should operate near threshold for full utilizing the mode selecting ability of RR. Great than 97% smooth pulses, with 17 mJ, are obtained in experiment.