The invention discloses a large-span T-type steel structure segmental beam mounting and closing method for an isle-connecting bridge. The method comprises the following steps that a, water tank balance weight systems are installed at the cantilever ends of adjacent T-type steel structure segmental beams respectively, and a bridge floor crane is preserved at the top of one segmental beam; b, a closure block is hoisted by the bridge floor crane, the plane position and elevation of the closure block are adjusted to be within the designed parameter standard, and the closure block is locked temporarily; c, jacking force is applied to the segmental beams on the two sides of a closure opening, the bridge profile of the closure block and the closure segment is adjusted, and finally the closure block is locked; d, wet joint vertical formwork pouring and closure strand stretching are completed. According to the method, mounting and closure of the large-span T-type steel structure beams with thespan above 150 m can be guided, the connecting stability and safety of the T-type steel structure segmental beams are effectively improved, investment of offshore large mounting equipment is reduced,the construction cost is reduced, the construction efficiency is improved, and the precision requirement of bridge closure mounting is met.