Abstract: A new algorithm of partial least square (PLS) artificial neural network (ANN) and back propagation (BP) in combination was applied to the simultaneous spectrophotometric determination of tryptophan,tyrosine and phenylalanine in amino-acid compound injections without separation.The paralysis in the ANN training was avoided by the improved back propagation algorithm.The simple transformation for target vactors was proposed.The network parameters were optimized by uniform design and over-fitting was avoided.