The SHG from a colliding pulse mode-locked (CPM) unstable resonator Nd:YAG laser with a antiresonant ring structure is obtained by using intracavity frequency doubling of BBO crystal. The SHG energy conversion effiency of 50. 4% is measured . We demonstrate theoretically and experimentally that the conversion efficiency of intracavity frequency doubling is higher than that of extracavity one in this kind of laser using BBO crystal.