Abstract: The optical constants of thin film are decisive factors for its optical properties. A simplified numeric algorithm expression for solving transcendental equation of optical constants was obtained by analyzing the test principle of ellipsometer. A Microsoft Windows based calculating program for optical constants of thin films with “Solve Block” module of MathCAD by ellipsometer was developed. It can be used to calculate refractive index and thickness of thin films with little absorption or even no absorption as its feature. The results of practical operation indicate that this method has the advantages of better accuracy, high precision, high speed, and universal usage. It can be used in online test with ellipsometer during the process of film preparation.