A set of space-dependent nonlinear coupling rate-equations has been established todescribe a CW self- frequency- doubling NYAB laser. Using these equations, the characteristics of the LD-pumped NYAB CW laser operating at 1.06 μm and 0.53 μm are studied by computer calculation. The influences of the spatial distribution of pump beam and laser beam, the optimum length of crystal and the optimum transmission of output mirror on laser output power have been discussed. Some design criteria. for experiments.