Soft cofferdam for bridge construction and construction method

The invention relates to a cofferdam for bridge construction, in particular to a soft cofferdam for bridge construction. A soft enclosure structure is arranged on one side of the cofferdam, and water-proof enclosure structures are arranged on the other three sides of the cofferdam. The soft enclosure structure comprises a fixing part, waterproof cloth and sandbags, the water-proof enclosure structures on the two adjacent sides of the soft enclosure structure are relatively fixed to the fixing part, one side edge of the waterproof cloth is fixed to the fixing part, and the other three side edges of the waterproof cloth are connected with the sandbags. When the waterproof cloth is in a working state, the waterproof cloth is divided into a first bending part and a second bending part, the first bending part is attached to the fixing part, the first bending part and the two waterproof enclosure structures adjacent to the fixing part are sealed, and the second bending part is attached to the bottom of a river outs