The strata structure of sea cofferdam in south shore of Su′ai Tunnel is complex. The upper part is backfill and mucky soil, the middle part is sand layer, and the bottom part is stronglyweathered and moderatelyweathered rocks with high strength, and there are a lot of potential boulders in the groove. In order to solve the technical problems of diaphragm wall construction under complex conditions, i.e. easy collapse of upper soft soil layer, unstable lower boulders with high strength, difficult borehole drilling and sampling of inclined bedrock, and low efficient trenching machine, the construction technologies are optimized and improved. And then the construction technology of "prereinforcement of groove wall, grabbing of soft soil by trenching machine, dense drilling of boulder and impact crushing by the percussion hammer, weathered rock removing by rotary drill and trenching machine, and inclined bedrock drilling by rollerbit hole rig". Finally, by strictly controlling the construction management procedures, the parallel operation can be realized and the efficiency of diaphragm wall construction can be greatly improved. The results can provide reference for similar projects in the future.