Offshore oil development project relies on floating crane. On "HYSY201" deepwater pipe-laying crane vessel, the 4000 t all slewing floating crane is designed according to European standards. Its building and assembly are precision, difficult and complex, and the assembly quality directly affects the performance of the floating crane. The key technologies including assembly procedure, precision measurement, virtual assembly, tooling design, and lifting are introduced in detail. The general assembly process for such floating cranes are discussed.