An energy conversion efficiency of 65.3% for second harmonic generation (SHG) from 1.06 μm to 0.53 μ m has been achieved by intracavity frequency doubling (IFD) in LiB 3O 5(LBO) crystal and a colliding pulse mode locked (CPM) Nd:YAG pulsed laser with a convex antiresonant ring (ARR) unstable resonator. For a fundamental wave with 20.2 mJ energy, 10 ps duration and a energy stability of 96.4%, a green output energy of 13.2 mJ was obtained. The energy conversion efficiency in IFD is much higher than that in extracavity frequency doubling (EFD) which is only 33.87%.