An optical device for corneal measuring includes a light source module, a first optical module, a second optical module including a reference mirror, a light splitter and an image analysis unit. The light of the light source module is transmitted to the first and second optical modules through the light splitter. The light is transmitted to a cornea through the light splitter and the first optical module and reflected by the cornea to form a first light, the light is transmitted to the reference mirror through the light splitter and reflected by the reference mirror to form a second light. The first and second lights are transmitted to the light splitter and the image analysis unit. The reference mirror moves along a first direction, and when the first light and the second light interfere with each other, a relative optical path length is obtained.