Abstract: A zoom beam expansion system for OPO(optical parametric oscillation) laser rangefinder transmitter was designed. The system adopted two-stage laser beam expansion, which compressed the divergence angle of OPO laser beam for the twice. The primary laser beam expansion was variable magnification beam expansion, and the secondary laser beam expansion was constant magnification beam expansion, so that the divergence angle of the output beam could be continuously adjusted to realize the wide range ranging of the laser rangefinder and improve the human-eye safety in the laser ranging process. The divergence angle of the output beam of OPO laser is 6 mrad, the diameter of the output beam is ϕ7 mm, and the beam expansion rate is 6 to 12 times, among which the variable magnification beam expansion rate of the primary laser is 2 to 4 times, and the beam expansion rate of the secondary laser is 3 times. The designed optical system meets the requirements of large-aperture beam incidence, and has the function of variable magnification beam expansion, which break through the limitation that the existing variable magnification beam expansion system can only achieve small-aperture beam incidence. The total length of the finally designed optical system is 285 mm, which is simple in structure and conducive to engineering.