The invention discloses a TMCP type marine austenitic stainless steel composite plate and a preparation method thereof and relates to the technical field of bimetal composite manufacture. The preparation method comprises blank preparation, surface grinding, isolating agent brush-coating, assembly blank sealing, electron beam soldering and sealing, heating, rolling and cooling, straightening, and cutting separation. The TMCP type marine austenitic stainless steel composite plate effectively solves the problem of corrosion resistance of the composite austenitic stainless steel in the composite plate, and in the blank preparation process, the steps of drilling and vacuuming are avoided. The preparation method has simple processes and a high rolling yield.