The offshore floating production facilities usually use multiple mooring legs to connect the upper platform facilities with the seabed anchor piles. The water depth of more than 1500 meters will bring unprecedented challenges and difficulties to the mooring leg pre-laying work. In this paper， based on the engineering practice of the semi-submersible platform mooring leg pre-laying work in the west of the South China Sea， the mooring legs are lowered by the method of dynamic positioning system， active heave compensation offshore crane and 500 ton anchor handling winch. Based on the connection between the anchor head chain and the anchor pile， the placement of polyester cables in the middle section of the mooring legs， and the wet storage of the mooring legs， this paper conducts a thorough analysis and summary of the key points and difficulties in the construction of anchoring handling vessels.