Intracavity frequency doubling with (beta) -BaB2O4 (BBO) crystal in a passively mode-locked unstable confocal resonator Nd:YAP (Nd:YALO3) pulsed laser is described. Without any amplification and pulse selection, a powerful quasi-single psec pulse at 0.54 micrometer with an energy greater than 20 mJ and power density 5 GW/cm2 was obtained by totally passive mode locking for the first time. Energy conversion efficiency 81% from 1.08 to 0.54 micrometer was achieved. The technique of intracavity second harmonic generation (ISHG) leads to stable output energy of SHG with an energy stability of 95.96%. This has a higher stability than that of fundamental wave with an energy stability of 92.78%.