Sketch to show the scenario of space weather forecasting for SEP events. The spirals correspond to interplanetary magnetic field lines, along and across which the SEPs transport diffusively in the solar-terrestrial space. The realistic physical parameters observed before a specific solar event by the in-situ spacecraft can be used to quickly determine the parallel and perpendicular mean free paths, with which we can further obtain the intensity and evolution of SEPs at a particular location in the interplanetary space.