To solve existing difficulties in the current laser ranging receiving optical system, that is, the system has sufficient light input while ensuring its low manufacturing cost and light miniaturization, a large relative aperture receiving optical system is analyzed and designed based on the Lagrange invariant theory. The proposed system uses a six-piece Galilean standard spherical mirror, and an avalanche photodiode (APD) with a photosensitive surface diameter of 0.5 mm, to detect an echo light signal. The optimized system consists of an entrance pupil diameter of 50 mm, a relative aperture of 1∶0.9, a total system length of 114.9 mm, and a Lagrange invariant of 125 mrad·mm, which not only obtains sufficient light input, but also addresses low-cost miniaturization requirements. To reduce the stray light influence, the lens hood and barrel fence structure are further designed. Stray light simulation tracking results indicate that the stray light suppression outside the 20°‒85° off axis angle field of view meets the laser ranging system requirements. Under the premise of receiving an optical system with a Lagrange invariant of 125 mrad·mm, the ranging range comprehensively increased by 21 times after adding the optical system. This confirms that the receiving optical system has significant application value.