The invention discloses a floating crane for overwater hoisting. The floating crane comprises a floating bottom frame, a vertical frame and a crane, at least two sets of longitudinal force bearing floating bodies which are arranged side by side in parallel, and the same-direction end of each longitudinal force bearing floating body is connected with a transverse connecting floating body, each transverse connecting floating body and each longitudinal bearing floating body are formed by splicing at least two steel protecting cylinders with two closed ends in an end-to-end mode, the vertical frame is arranged at the top end of each longitudinal force-bearing floating body, and the crane is arranged at the top end of the vertical frame. The floating crane for overwater hoisting has the structure of utilizing an idle steel protective cylinder as a floating body structure, and has the advantages of being easy to assemble and disassemble, low in cost, remarkable in construction benefit and the like. The method for assembling the floating crane for overwater hoisting comprises the following steps of S1 manufacturing the floating bottom frame; S2 building the stand columns; S3 fixedly mounting the crane on the top ends of the stand columns. The method is easy to assemble and fast to dismount.