The experiment presents an external ring cavity for frequency doubling, which is used to generate 461 nm blue radiation, with PPKTP as a laser frequency doubling crystal. By the incorporate ring cavity and the locking circuit system, the output power of 461 nm reaches the maximum. More than 208 mW of the second harmonic generation is gotten from 350 mW IR input power in the experiment. The coupling conversion efficiency is 73.3%. By analyzing and amending the loss error the reflectivity of coupling mirror is optimized. The effect of two different reflectivity coupling mirrors, PPKTP crystal’s linear absorption effect and thermal lens effect are discussed.