A method for obtaining a phase of a micro relief structure in an optical security device with full parallax diffraction optical variable image, the optical security device comprising: a base layer; and an optical assembly at least partly covering one of surfaces of the base layer, and comprising the micro relief structure, so that the optical assembly presents a full parallax image under action of the micro relief structure when a beam of white light illuminates on the optical assembly, the method comprises the following steps:- 1) decomposing an original image to be presented into a plurality of image frames corresponding to various observing directions; 2) obtaining a complex amplitude of an object plane according to amplitudes of the image frames and direction coefficients related to the image frames; 3) performing a fresnel reverse transformation on the complex amplitude of the object plane, and extracting a phase of the complex amplitude of object light at surface of the optical assembly obtained after the fresnel reverse transformation to obtain a phase of the micro relief structure; 4) obtaining a complex amplitude of the micro relief structure according to the phase of the micro relief structure; 5) multiplying the complex amplitude of the micro relief structure by a predesigned complex amplitude of a phase plate, and performing the fresnel transformation, decomposing the obtained result into a plurality of image frames corresponding to various observing directions, then taking amplitude of the complex amplitude of each image frame in the plurality of image frames as 1, and keeping the phase, and then multiplying the same by amplitude of frames of the original image to obtain a new complex amplitude of the object plane; and 6) performing the fresnel reverse transformation on the new complex amplitude of the object plane, and extracting a phase of the complex amplitude of object light at surface of the optical assembly obtained after the fresnel reverse transformation, to obtain the phase of the micro relief structure. a base layer; and an optical assembly at least partly covering one of surfaces of the base layer, and comprising the micro relief structure, so that the optical assembly presents a full parallax image under action of the micro relief structure when a beam of white light illuminates on the optical assembly, the method comprises the following steps:- 1) decomposing an original image to be presented into a plurality of image frames corresponding to various observing directions; 2) obtaining a complex amplitude of an object plane according to amplitudes of the image frames and direction coefficients related to the image frames; 3) performing a fresnel reverse transformation on the complex amplitude of the object plane, and extracting a phase of the complex amplitude of object light at surface of the optical assembly obtained after the fresnel reverse transformation to obtain a phase of the micro relief structure; 4) obtaining a complex amplitude of the micro relief structure according to the phase of the micro relief structure; 5) multiplying the complex amplitude of the micro relief structure by a predesigned complex amplitude of a phase plate, and performing the fresnel transformation, decomposing the obtained result into a plurality of image frames corresponding to various observing directions, then taking amplitude of the complex amplitude of each image frame in the plurality of image frames as 1, and keeping the phase, and then multiplying the same by amplitude of frames of the original image to obtain a new complex amplitude of the object plane; and 6) performing the fresnel reverse transformation on the new complex amplitude of the object plane, and extracting a phase of the complex amplitude of object light at surface of the optical assembly obtained after the fresnel reverse transformation, to obtain the phase of the micro relief structure. 1) decomposing an original image to be presented into a plurality of image frames corresponding to various observing directions; 2) obtaining a complex amplitude of an object plane according to amplitudes of the image frames and direction coefficients related to the image frames; 3) performing a fresnel reverse transformation on the complex amplitude of the object plane, and extracting a phase of the complex amplitude of object light at surface of the optical assembly obtained after the fresnel reverse transformation to obtain a phase of the micro relief structure; 4) obtaining a complex amplitude of the micro relief structure according to the phase of the micro relief structure; 5) multiplying the complex amplitude of the micro relief structure by a predesigned complex amplitude of a phase plate, and performing the fresnel transformation, decomposing the obtained result into a plurality of image frames corresponding to various observing directions, then taking amplitude of the complex amplitude of each image frame in the plurality of image frames as 1, and keeping the phase, and then multiplying the same by amplitude of frames of the original image to obtain a new complex amplitude of the object plane; and 6) performing the fresnel reverse transformation on the new complex amplitude of the object plane, and extracting a phase of the complex amplitude of object light at surface of the optical assembly obtained after the fresnel reverse transformation, to obtain the phase of the micro relief structure.