The invention provides a calculating method for a long-and-short pile combined support structure. When the long-and-short pile combined support structure is designed, for a local unit model which is located at the selected weakest and most dangerous section and contains N

piles, the soil pressure at the part above the pile bottoms of short support piles is jointly borne by long support piles and the short support piles, and the soil pressure at the part below the pile bottoms of the short support piles is completely borne by the long support piles independently; 2N

boundary conditions are formed by presuming that the bending moment and shearing force at the pile bottom of each pile are zero, N

boundary conditions are formed by presuming that the bending moment at the pile top of each pile is zero, N boundary conditions are formed by presuming that the pile topes of all the piles are equal in displacement, one boundary condition is formed by presuming that top beams in the local unit model are in a balanced state, and then all the boundary conditions are combined to form 4N

boundary conditions; and a local unit model control equation set based on the elastic resistance method theory is solved through a shooting algorithm based on the Broyden method iteration, and thus the displacement, rotating angle, bending moment and shearing force of a pile body of each support pile are obtained. According to the calculating method, the concept of the local unit model is provided, and only the most dangerous area needs to be calculated; the design safety is achieved, and meanwhile, the spatial effect of the combined action of the long-and-short pile combined structure can be considered, so that the physical concept is clear, and calculation is simple; and the shooting algorithm based on the Broyden method iteration is not sensitive to the setting of an initial value, the convergence speed is high, and the stability is good.