An iteration method is presented to reconstruct planar waveguide refractive index profile from mode indices spectra. In the current method, a numerical process is employed to solve the Helmholtz equation, the “turning point” positions of all the propagation modes are adjusted in sequence from the lowest to the highest by iteration, and repetitions of the iteration are performed to eliminate the effect of refractive index profile variation beyond each “turning point” on the corresponding “turning point” position. The convergence of “turning point” position over iteration in this process is briefly illustrated by means of the WKB principle. A demonstrative reconstruction is made for a planar waveguide with exponential refractive index profile, and results show that the current method can be used to reconstruct planar waveguide refractive index profile with high accuracy. Analysis indicates: this iteration method is expected to be superior in both self-consistence and accuracy to the inverse WKB method.