Abstract: In view of the surface settlement caused by excavation and dewatering of deep foundation pit, the con? cept of ground loss in random medium theory was introduced, the calculation formula for the surface settlement caused by foundation pit excavation was deduced, and the formula for surface settlement caused by changes of unit weight and penetration was deduced and improved, the determination of key parameters of random medium theory applied in calculation of foundation pit excavation was discussed by taking the effect of frictional resistance at support structure-soil interface on settlement into consideration, and a contrastive analysis of the calculation results by the methods proposed in this paper and reference10 and the measured settlements was conducted. The results show that the surface settlement near foundation pit calculated by the method proposed in this paper is closer to the measured ones, the surface settlement caused by excavation and dewatering of foundation pit can be predicted more accurately using the proposed method; compared with the traditional methods, the calculation accuracy of this method is much higher within the scope of 1.5 times excavation depth while the calculation accuracy would be further improved beyond the scope of 1.5 times excavation depth by considering the effect of specific water retention of soil mass and frictional resistance at the side of support structure; as for the open foundation pit with support structure and dewatering in the pit, good prediction of surface settlement can be achieved with the calculation method proposed in this paper.