The invention discloses a Young modulus micro elongation measuring device based on optical grating diffraction. The measuring device comprises a vertically arranged bracket, wherein the bracket is provided with a rotating shaft; the rotating shaft is provided with a rotating arm and an optical grating both of which can rotate around the rotating shaft; the rotating arm is provided with a parallel light source; the bracket is provided with a telescope for observing diffracted light from the optical grating; the telescope and the parallel light source are respectively positioned at the two sides of the optical grating; light emitted by the parallel light source points to the optical grating; the rotating arm is connected with the lower end of a test sample; the upper end of the test sample is fixed; the optical grating is connected with a vernier rotating disc; the vernier rotating disc can rotate along with the optical grating; the bracket is provided with a graduated disc; and the rotating angle of the optical grating can be read under the matching action of the vernier rotating disc and the graduated disc. The invention also provides a Young modulus micro elongation measuring method based on optical grating diffraction. Thus, Young modulus micro elongation is measured by using the optical grating diffraction theory, thereby improving the magnification and lowering the operation difficulty of experiments.