The invention discloses a horizontal expansion device for a bridge detection operating vehicle. The horizontal expansion device comprises a boom assembly which is of a truss type structure and a chain transmission device arranged at the lower part of the boom assembly, wherein the boom assembly comprises basic arms and expansion arms; the chain transmission device comprises symmetrically arranged transmission chains, a chain wheel box, guide chain wheels I symmetrically arranged on the chain wheel box, symmetrically arranged guide chain wheels II and driving chain wheels; the transmission chains are respectively meshed with the three types of chain wheels in a v-shaped manner; two ends of the transmission chains are respectively fixed at the lower parts of the expansion arms of left and right tensioning devices; the two driving chain wheels are mounted on one transmission shaft and driven by one hydraulic motor; and the chain transmission device is symmetrically arranged at the front and rear positions of the boom assembly, the driving chain wheels are mounted on the same transmission shaft, and the transmission shaft is connected with an output shaft of the hydraulic motor, so that the phenomenon of unsmooth expansion motion and even the risk of breakage of the chains due to mutual inclination of central axial lines of the expansion arms and the basic arms can be avoided.