In this paper the electric field distribution in optical coating is calculated by Fresnel coefficient matrix method. On the bases of the distribution of fields, the design for a conventional antireflection coating, high reflective coating and an interference filter have been improved. These examples show the relationship between field distribution and optical losses in thin films.With the distribution of the fields modified or the defect reduced in the layers and the interfaces, optical losses can be restricted. Thus, the successful method is provided for designing excellent optical coatings with lower losses.