The invention discloses a floating installation method for an upper assembling block of a tension leg platform. The floating installation method includes the following installation steps: 1, pulling a lower floating body to an installation sea area, and fixedly connecting the lower floating body; 2, connecting the upper assembling block with a power positioning ship, and dryly pulling the upper assembling block to the installation sea area; 3, monitoring the movement of the lower floating body and the movement of the power positioning ship; 4, when the installation requirement is met, starting ship entering work; 5, after the lower floating body is submerged to be at the set immersion depth, adjusting the position of the upper assembling block and the position of the lower floating body, and preparing butt joint work; 6, enabling the lower floating body to continuously float till the lower floating body is in contact with the upper assembling block, transferring the weight of the upper assembling block to the lower floating body, and enabling the ship to get out of the lower floating body and be driven out of the installation sea area; 7, fixing the upper assembling block and the lower floating body, and continuously and wetly pulling the upper assembling block and the lower floating body to a final working sea area for other-part installation. According to the floating installation method, after the upper assembling block and the lower floating body are independently built, floating installation is completed on the sea; influences of a building site on assembling block installation are greatly reduced, and the on-sea connecting debugging and installation time is saved.