Second-stage engineering cofferdam construction method based on first-stage engineering longitudinal steel sheet pile cofferdam

The invention discloses a second-stage engineering cofferdam construction method based on a first-stage engineering longitudinal steel sheet pile cofferdam, which comprises the following steps: after the first-stage engineering construction is finished, dismantling an upstream transverse cofferdam and most downstream transverse cofferdams of the first-stage engineering; a downstream transverse cofferdam close to the shoreside part of the first-stage project and a longitudinal steel sheet pile cofferdam of the first-stage project are reserved; a downstream transverse cofferdam, a steel trestle and a reserved first-stage longitudinal steel sheet pile cofferdam of the reserved first-stage project close to the shoreside are used as construction roads, upstream cofferdam and downstream cofferdam construction of a second-stage project is conducted, and the construction roads are connected with a second-stage project longitudinal reinforced concrete cofferdam constructed by the first-stage project to form a second-s