Basing on experiments and theories, we study frequency doubling in a KTP crystal on Nd∶YAG laser by the resonant external ring cavity. The Nd∶YAG laser average power is 50 W with Q-switch and its frequency is 1005 Hz. In the experiment we obtain 0.532 μm green light output with 31.4% light conversion efficiency when 1.064 μm input average power is 35 W. With the experimental results, we analyze the experimental results for the characteristics of ring cavity frequency doubling and summarize its strength and weakness. We compare fundamental frequency laser and second harmonic generation in terms of laser light quality. The thesis provides some pictures of beam focuses acquired with CCD, compares some experimental results with those pictures. We put forward a theory of the ring cavity frequency doubling in this paper.