Abstract: The structure of preloading filling spiral case can take full advantage of the strength of steel, lower stress of outside concrete, reduce the ratio of reinforcement and make the structure of the spiral case safe, reliable. Hydraulic structure of preloading filling spiral case has been widely applied due to its excellent advantage in the strength of steel, low stress of concrete outside, and the low ratio of reinforcement with safety and reliability. In view of the complex shape of the spiral case, it is impossible to exactly ensure that all parts work under the ideal situation of design load, and the deformation cannot all inflation at the same time. Moreover, the action of axial force from the bulkhead under internal pressure has also posed a significant influence on spiral case deformation in the construction process of a constant internal pressure spiral case. Therefore, the initial gap of spiral case usually presents local closed and non-uniform conditions. It is necessary to explore the initial non-uniform gap and contact behavior, in order to approaching an ideal situation of no gap, no energy transfers between spiral case and surrounding concrete. The accurate evaluation of initial non-uniform gap is also a prerequisite of solution algorithm for preloading structure of water-filled spiral case. However, the constraint way on the construction phase needs to be considered because the initial gap value varies with the preloading water head and boundary conditions in the specific application sites. In this paper, a modified simulation algorithm based on the construction process is proposed to study the preloading water-filled spiral case of NZD project. The results show that the proposed method can effectively simulate the initial gap with non-uniform and local closed condition that caused by the asymmetrical structure of spiral case. The variation of initial gap strongly depends on the radial deformation of spiral case. Compared with the simplified method, this modified algorithm considers the gap and contact relationship, which is key factor to make both interaction and transfer force more complicated in previous studies. The deep analysis has carried out on the influence of different support forms, such as continuous and discrete support, and contact behavior on the initial gap during the calculation. There are important effects support form and contact on the specific value of the initial gap. Especially in the entrance section of spiral case, there is a 1.8 times difference, compared to that without considering the contact conditions. The reason is that the contact behavior of the initial can increase the sideslip of structure when preloading filling spiral case. The findings demonstrate that the simulated results are more reasonable after considering the contact conditions of the initial gap, compared with the measured data from hydraulic structure sites.