The invention relates to a dual-row cofferdam and relative method, which is characterized in that: a row of steel plate post cofferdam is arranged at the outer side of steel tube post cofferdam, and aearthwork filling is processed between two rows, and it strengthens them to form the waterproof curtain. The invention utilizes the large rigidity of steel tube post and the better waterproof property of steel plate post to combine them, with lower cost and simple process.