Provided is a method for preparing a stainless steel composite plate for a TMCP bridge. The stainless steel composite plate for the bridge, having excellent metallurgic bonding, is prepared by means of the steps such as blank selection, surface treatment, isolating agent brushing, blank assembly, electronic beam sealing and welding, heating, rolling, straightening, cutting and plate separation. In the method, sealing and welding are directly carried out in a vacuum chamber environment by using electronic beams, drilling, vacuumizing and other procedures in the traditional technology are omitted, and accordingly the vacuum degree is better ensured. By means of the TMCP process, excellent mechanical performance of substrate bridge steel and quick cooling after rolling are ensured, intercrystalline precipitation of a coating material is controlled, and excellent corrosion resistance of the coating material is ensured. A designed low-carbon blank is used on the substrate. The prepared composite plate has a low carbon equivalent in the substrate, has excellent welding performance and is convenient for onsite construction; and in addition, the composite plate can obtain excellent comprehensive performance without heat treatment.