The invention discloses a vertical response calculating method of a three-slide-block monorail flexible rocket sled. Bearings of the rocket sled are analyzed, and a rocket body is dispersed into finite element beam models; structural vibration, caused by unsmoothness of a rail, of the rocket sled and the rail is taken into consideration and a Hertz nonlinear contact model is used to calculate collision contact forces born by the rocket body in different time; and the whole-course vertical dynamic response of a three-slide-block monorail rocket sled system within given time is calculated by utilizing a central difference value method. In the calculation process, flexible deformation characteristic of a rocket sled body and a quality reduction effect, caused by fuel ejection, of the rocket sled system are taken into consideration, the nonlinear contact problem caused by that the rail is unsmooth is also considered, and thus, a more reliably dynamic response result can be obtained.