ELID grinding technology is used widely in the precision grinding of hard and brittle materials. The nature of passivating film play an important role in ELID mirror grinding. Spring’s stiffness of passivating film was researched from theory and simulation and experiment. And the theory of the dynamic stiffness of spring was proposed, based on this theory minimum mechanism of roughness of ELID grinding was proposed further, and experimental verification was done. Results show, the spring stiffness of passivating film is change with the grinding paramenter, and when implementing the precision grinding, the elastic deformation of oxide film reduces the exposure and grinding depth of the grinding grains, accordingly, ameliorating the grinding surface quality.