Abstract A spiral case with its steel spiral case (SSC) being embedded in reinforced concrete under a pressurized condition is called a preloaded filling spiral case structure (PFSCS) in a hydropower plant. As a steel-concrete composite structure, a PFSCS is designed to work reliably. The non-uniform gap and contact nonlinearity between the SSC and the surrounding mass concrete have a great effect on the bearing mechanism of the composite structure. However, the description of the gap and contact nonlinearity in a PFSCS is a tough work. With the aim of efficiently describing the evolution process of the non-uniform gap and contact nonlinearity, we performed an experimental investigation and proposed a novel numerical simulation technique for structural finite element analyses (FEA) of PFSCSs. In the technique, the gap and contact nonlinearity as well as the construction process and operation process of a PFSCS are taken into account. A friction-contact model is used to simulate the sliding of the SSC against the concrete. A plasticity damage model is employed to describe the concrete. The development of the gap, contact status between the steel liner and the surrounding concrete, stresses of the steel liner and the steel bars, as well as the concrete cracking time and crack pattern, are presented in this work. The FEA results agree well with the experimental results. The agreement provides evidence that the applicability and competence of the proposed technique are valid and satisfactory.