A general theoretical of continue wave optical parametrical oscillator that KTP crystal pumped by an Ar+ laser at 514.5nm was analyzed.It was discussed that the angle-tuning curve,the effective nonlinear coefficient and walk-off angle when the KTP crystal pumped with the phasematching of the typeⅡ,the pumped threshold power was counted under this condition.The parameter of the apparatus was well designed,the cutting angle of KTP crystal was conformed θ=67°，φ=0°,a signal wave with 1.55μm center wavelength was presented.