In the paper, a new technology for selecting single longitudinal mode (SLM) of solid state laser by phase delay of a birefingent crystal is proposed. Its principles of loss modulation are analysed. Based on the semiclassical theory, dynamic equations of light field are derived when several longitudinal modes compete in a cavity. Its ability for selecting mode is calculated. The oscillation of SLM is experimentaly demonstrated in a solid state laser by this way. The output power of 44 mW and stability of ±3% are obtained by using KTP crystal. It is shown that this technology is preferable to selecting SLM in diode pumped solid state laser with mid-lower output power.