Underground power pipelines are generally buried in the soil in the form of box culverts. When such existing pipelines are involved in underground construction, the in-situ support protection method is generally used to ensure the stability of the box culvert during the construction process. The determination of the distance between the support points mainly depends on the engineering experience. In this paper, considering the cross-sectional properties of the box culvert and the settlement effect in the soil, the mechanical models of the middle-span and the side-span of the box culvert are respectively established, the vertical displacement of the box culvert is calculated, and the allowable support points are deduced. Based on the maximum spacing value, a calculation method for the spacing between the support points of buried box culverts under in-situ support protection is proposed, and the accuracy and engineering suitability of the calculation method are verified by a case of the in-situ protection project of large-diameter pipe culverts at Yunnan Road Station of Nanjing Metro Line No. 5. The full-span longitudinal deformation pattern and parameter sensitivity of the buried box culvert are further analyzed through numerical simulation. The research shows that: the spacing between the supporting points of the box culvert side span is 8 m, the spacing of the middle span is 8.73 m, and at least 12 supporting points need to be set for the whole span. Within the range of three times of parameter magnification, with increase of reinforcement depth, width and soil compressive modulus, the displacement of the box culvert gradually decreases.