The invention belongs to the field of civil engineering, and particularly discloses a method for obtaining true mechanical property and constitution of underground concrete after erosion by sulfate. The method comprises the following steps of firstly, delaminating eroded specimens in different erosion ages along the erosion depth direction, and assuming that the concentration of sulfate ions in each layer is uniform; obtaining a rigidity-strain relationship curve of concrete by a derivation method of lagrange interpolating polynomial according to a load-strain curve of specimens in a test group in the erosion ages and a load-strain curve of uneroded specimens in a contrast group in the same ages; according to a rigidity parallel formula of concrete, obtaining the relationship curve of concrete in certain layer; finally, according to numerical integration, obtaining a stress-strain relationship curve at different concentrations of sulfate ions. The method has the advantages that the operation is simple and convenient, and the calculation is simple; the bearing capacity of concrete part after erosion can be accurately estimated, and be evaluated and maintained, the safety in runningis ensured, and the service life is prolonged.