Device to read/write data from optical storage units have polarization insensitive character at two separated wavelength. Based on a key four-layer structure and some matching layers, an initial thin film stack system is constructed. After optimized alternately by simplex and conjugate graduate algorithm, a double-wavelength polarization insensitive beam splitter with splitting ratio of R : T=50:50 at 650-nm wavelength and T > 93% at 780-nm wavelength is gained. The design result shows that the difference between reflectivity of P and S light around wavelength range of 630–670 and 760–800 nm with incident angle of 40o–50o is all very little. That indicates our design controls the polarization deviation well at two separate wavelengths with a reasonable range for both wavelength and incident angle.