The invention discloses a mounting and constructing method for a deepwater comb-shaped slide cross beam, which is based on and integrates short section matched prefabrication, rolling pre-assembly and underwater mounting and construction, and solves the difficult problems in the traditional construction method that the construction risk is higher as long-time diving operation of divers is required, besides, the elevation and axis alignment are difficult to control and the requirement for high-precision mounting of rails on the slide cross beam cannot be met as the slide cross beam is simultaneously influenced by storms and water flows. According to the method, the prefabricating line type, the pre-assembly line type and the mounting line type of the slide cross beam are dynamically adjusted and controlled in a real-time manner, no error accumulation is generated during the whole beam construction, so that the beam is precise in size and line type; the method has the advantages of high technological content, high degree of factory-like and mechanized operation, environmental protection and the like, and facilitates the organic integration of the advanced computer technology and electrical and mechanical manufacturing technology with the slipway slide construction technology.