Abstract: In the paper,the merits of laser diode pumped solid state ring laser are introduced. The gyroscope can be made in solid-state,the quality of the beam can be improved and the dead lock can be reduced. But because of the multi longitudinal mode the precision of the gyroscope is decreased. Methods of longitudinal mode selection are discussed. Meantime these methods are compared,dispersion,F-P etalon,composite cavity etc. The con clusion is that F-P etalon is the most effective method to realize the single-longitudinal mode and others will be validated in the experiments.