The analysis on the seawater refractive index which increases the error of oil film thickness measured by differential laser triangulation are taken to discover the relationship between the change of refractive index and error. Accordingly a method based on two-dimensional curved surface fitting is proposed to compensate the error. The data of thickness are measured in the water of different refractive indexes and the quadratic polynomial curved surface fitting is adopted to formulate the error, thickness and refractive index. Then the measurement of thickness is compensated by using the above the formula and the refractive index. The experiments taken on different refractive indexes show the thickness measurement compensated is closer to the true value and the error is minimized.