The invention discloses a buoyancy mounting method for blocks. The method comprises the following steps of: (1) arranging block jointing devices on struts of the blocks to be arranged in advance; (2) transversely mooring a power positioning ship to a wharf, wherein the heights of sliding ways of the wharf are ensured to be consistent with those of sliding ways on a main deck of the power positioning ship; (3) dragging the blocks onto the power positioning ship for fixed welding, wherein the power positioning ship has self-propulsion capability and power positioning capability; and (4) propelling the power positioning ship to conduit supports to be arranged, propelling the power positioning ship to notches of the conduit supports, jointing the blocks and the conduit supports when the power positioning ship reaches the jointing positions of the blocks, and propelling the power positioning ship out of the notches of the conduit supports to finish the buoyancy mounting of the blocks after the blocks are jointed. The method can be widely applied to the entire buoyancy operation of the blocks, in particular to block mounting under severe environmental conditions in the south sea of China, the development cost of oil and gas fields is saved, and the efficiency is improved.