Focal length measuring device adopting symmetrical grating

The invention discloses a focal length measuring device adopting a symmetrical grating. The focal length measuring device adopting the symmetrical grating comprises a laser device, a collimation and beam expanding lens, a to-be-tested lens, a first grating, a second grating, a receiving screen, a imaging objective lens and a charge coupled device (CCD) camera, wherein the laser device, the collimation and beam expanding lens, the to-be-tested lens, the first grating, the second grating, the receiving screen, the imaging objective lens and the CCD camera are arranged on the same optical axis in sequence. The second grating is a symmetrical grating. Before the to-be-tested lens is placed in, laser emitted by the laser device penetrates through the collimation and beam expanding lens to form collimation light, the collimation light lights on the to-be-tested lens, penetrates through the first grating, and forms a taber image at the position of the second grating, the taber image superimposes with the second grating to obtain a moire fringe, the formed moire fringe is received by the receiving screen, and images on the CCD camera through an imaging lens, the image received at this time is marked as a moire fringe L1, a received image after the to-be-tested lens is placed in is marked as a moire fringe L2, and focal length of a tested system can be obtained by calculating an inclined angle between the moire fringe L1 and the moire fringe L2. The focal length measuring device adopting the symmetrical grating has the advantages of being simple in structure, high in accuracy, easy to achieve, and capable of being used in accurate measurements of a long-focus system.