The mode equation of graded wavequide (WKB integral equation) was divided into many segmental integrals and their terminal values points were set at the turning points of the same mode at different wavelengths. When the differences of the wavelengthe were small enough, the deviations for the corresponding turning points are also small enough, the broken lines approximations oould be made in these segmental integrals. Then, we theoretically obtained a recurrence formula for calculating the refractive index profile. The profile can be finally determined by picking up the smoothesii one. This method is especially applicable for monomode waregnlde and it is urineoessary to presuppose the function type of the profile. Computer aimulations are made for model waveguides with the profiles of hypertolio secant and parabolic types in this paper and the caloulation had proved that the new method is with the precision of 10-3 or even higher. The more segments were divided, the higher precision oould be reached.