The invention relates to processes for constructing cofferdams, in particular to a method for constructing a split type cofferdam. The method includes that the split type double-wall steel cofferdam is processed, assembled and formed on a shore and comprises two 'U'-shaped cofferdams; the split type cofferdam is integrally transported to be close to a pier position; then the split type cofferdam is disassembled into the two 'U'-shaped cofferdams, and the two 'U'-shaped cofferdams are respectively conveyed into the pier position from an upstream and a downstream of a pier; and then the two 'U'-shaped cofferdams are integrally connected with each other in a closing manner, and planes of the cofferdam are precisely positioned, so that the split type cofferdam is sunken to meet the requirement of a designed elevation. The method for constructing the split type cofferdam is wide in application range, can be used for constructing common deepwater foundations of bridges, and is particularly applicable to maintaining and strengthening construction for old foundations of bridges, the construction speed is high, influence of existing structures of the bridges is avoided, and operation is facilitated as compared with other methods for constructing double-wall steel cofferdams.