A construction method of a large multifunctional over-water construction platform comprises the following steps of docking a required floating body on the river bank side, suspending processed member units on the floating body, connecting and assembling the processed member units to form the platform, placing a pile driving device and steel pile casings on the platform in advance, then removing an anchoring system of the floating body, using a tugboat to drag the floating body to the designed pier position, pulling an anchor line to accurately position the platform, performing inserting and hitting operations on wading piles by means of a guiding device on the platform, wholly lifting the platform to perform system transformation after finishing inserting and hitting operations of the wading piles, fixing the platform on the wading piles, then continuously performing inserting and hitting operations on other steel pile casings, meanwhile performing drilling pile construction, constructing a cofferdam after finishing the drilling pile construction, and adjusting the platform elevation to enable the platform to serve as an inner supporting structure of the cofferdam. The construction method utilizes a construction process for floating body assembly and multifunctional platform transportation, the number of required large over-water devices is small, processing and installation on the bank are convenient, trestle construction in advance is not needed, and shortening of a construction period and reduction of the construction cost are facilitated.