<p>The stability of the construction process of fill slope strengthening projects is critical to the life safety of construction personnel. In this study, a slope stability analysis model for each backfilling-strengthening stage was established to clarify the backfilling-strengthening process of this slope type. Additionally, the integral expression of the slope stability coefficient in each backfilling-strengthening stage was derived based on the integral method. The influence coefficient kj of the overlying soil layer thickness of the anchor plate is proposed based on the actual stress characteristics of the anchor plate, and the optimal length calculation model of anchor plates, which satisfies the stability requirements of the backfilling-strengthening process, was established. Additionally, the stability analysis process for a fill slope reinforced by frame prestressed anchor plates with consideration of the backfilling-strengthening process is presented. The solution set for the optimal length of the anchor plates, obtained using the proposed method, does not only satisfy the overall stability requirements for fill slopes strengthened by frame prestressed anchor plates, but also satisfies the stability requirements for the backfilling-strengthening process. The proposed method can be applied to the practical engineering design of this slope type.</p>