Abstract: In reservoir banks in southwestern China, colluvial deposits with weak interlayers are widely developed. During reservoir impoundment, the weakening of saturated soil strength and the increase in pore water pressure can easily trigger sliding along weak interlayers, posing a serious threat to engineering safety. Traditional limit equilibrium methods cannot accurately couple the seepage and stress fields, and their reliability in identifying the critical sliding mechanism along polyline slip surfaces with multiple inflection points is limited. To address these issues, this paper proposes, based on the principle of virtual power, a method for obtaining forces on the slip surface by employing a node-element linked-list indexing technique to construct velocity discontinuities and to search for intersection points between the discontinuities and the finite element mesh. For the polyline sliding mechanism, a grouped dimensionality-reduction nested-loop optimization strategy is further developed, combining a full-coverage grid node enumeration in the variable space with multi-variable gradient method or single-variable golden-section search to handle multi-dimensional variable constraints and multi-extremum problems. Based on coupled seepage–stress finite element analysis, the seepage and stress fields under various impoundment conditions are obtained, and the evolution of the stability safety factor for rigid block sliding mechanisms with polyline slip surfaces is investigated. The results show that: (1) the proposed grouped nested-loop method can accurately capture the critical sliding mechanism; (2) the safety factor in the outermost loop, where the optimization variables are the entry and exit points, exhibits multi-extremum characteristics, and the combined search strategy ensures both global optimality and optimization efficiency. This study provides an efficient and reliable theoretical tool for stability analysis of reservoir bank slopes during impoundment, reveals the dynamic evolution of slip surfaces, and offers significant reference value for engineering hazard prevention.