A simplified polar coordinate method based on the non-cosine film thickness formula is proposed for characterizing film thickness distribution. Herein, control of the uniformity of optical film thickness formed by single source electron beam evaporation is studied. Simultaneously, the position of the mask plate is calculated. Compared to the traditional method of placing the mask directly above the evaporation source, the proposed polar coordinate method is used to calculate mask position, which is more conducive to controlling the uniformity of film thickness distribution. Considering an evaporated H4/MgF₂ combination as an example, the mask positions and shapes of high and low refractive index materials are calculated, and single layer films are prepared using these two materials. The measured spectral uniformity deviation is better than 0.3%, thus demonstrating the correctness and feasibility of the proposed method.