Two kinds of frequency doubling methods by using KTP crystal are introduced. The first is frequency doubling in a folded cavity, and the second is achievement of second harmonic generation in a resonant ring cavity. In the latter method, using an 1 kHz Q switched Nd∶YAG laser with an average power of 50 W as the input light, a green light with average power of 17.5 W and harmonic conversion efficiency of 35% is obtained. The laser used for mini focus of laser rapid prototyping is demonstrated.