In this paper a new theoretical mode of a multifunctional crystal laser wag reported. The coupling wave-rate equations wore estiblished by using slowly varying amplitude approximation. The NYAB lasersswas modeled with a computer and the theoretical results were obtained. The threshold value and peak power of the lasers were discussed. Then the NYAB laser multifunctional effect was studied with a gaint pulsed laser (λ=589nm) pumped, the output peak power at 530nm was 37.5kW. The theoretical results is coincided with experimental results. Considering the deviation between the theoretical results and experimental results, some proposal about improving multifunctional lasers were raised.