On the basis of the existing test data of high temperature mechanical properties of 1860 grade steel strands, a regression model of high temperature performance of steel strands that can be used for theoretical analysis and numerical calculation is obtained. Based on the non-stationary temperature field model in the fire of tall and large space buildings, the non-linear finite element numerical analysis method considering the time integral effect is used to establish the fire resistance numerical model of the large-span prestressed steel structure. Through the fire-resistance calculation example of Beam String Structure, the influence of different fire source positions on the fire-resistance of prestressed steel structures is discussed.