The invention discloses a floating transportation device and method for major diameter underwater water pipes. The floating transportation device for the major diameter underwater water pipes comprises the water pipes with lengths being 54-60 m and 138-140 m correspondingly, multiple balancing weights, and multiple air bags; the multiple balancing weights are fixed on the water pipes at intervals,head plates are separately fixed on the end heads of two ends of one of the water pipes, the multiple air bags are arranged along the two longitudinal sides of the water pipes at intervals, and multiple beams of I-shaped steel are connected end to end and are separately fixed under two sides of the middle portions of the balancing weights arranged at intervals. The method for the major diameter underwater water pipes comprises the following steps that (1), HDPE pipes are connected in a welded mode and the balancing weights are installed; (2), the head plates are installed and a closed water test is carried out; (3), the water pipes float and the air bags are installed; (4), the water pipes are flooded with water and sink; and (5), the air bags are deflated and the water pipes are immersed. According to the floating transportation device and method for the major diameter underwater water pipes, the quantity of stainless steel flanges and the cost of the construction material are reduced greatly, total time of butt joint of water stainless steel flanges is reduced, the cost of underwater construction of the water pipes is reduced, the construction period is shortened, and construction efficiency is enhanced.