Determining the potential failure surface plays an important role in basal stability analysis of braced excavations in soft clay by upper bound method. For traditional upper bound method, it is assumed that failure surfaces are only related with excavation width. However, for progressive search method, the critical failure surfaces can be evaluated by multi-iterations without assuming detailed shape of failure surfaces. And this method was successfully applied to slope stability analysis. Firstly, this paper introduced the step of basal stability analysis of excavation via multi-block upper bound method. Secondly, this paper illustrated the framework of progressive search method. Considering the characteristic of basal stability analysis of excavation, the generation of initial failure surfaces, the search boundary and the convergence criteria are reset. And some solutions are proposed to deal with the problems during search process. Moreover, this paper verifies the performance of progressive search method by case studies, and proposes conclusions about search length, number of pointsat the failure surface and number of searches. At last, this paper concludes that progressive search method is applied in basal stability analysis successfully. And compared to other upper bound method, progressive search method without assuming detailed shape of surfaces is easier to calculate.