The mediated coupling of surface plasmon polaritons (SPPs) by a parallel moving electron beam is demonstrated in this paper. The theoretical analysis shows that the electron beam excited spoof surface plasmon polaritons (SSPs) on the grating placed above the metal films play the role as the excitation source in the mediated coupling. The numerical calculations and particle-in-cell simulations demonstrate the significant advantages of the SSPs mediately coupled SPPs in contrast with that coupled by the parallel moving electron beam directly. The photo density of the mediately coupled SPPs reaches up to 106 per cm2 for the electron beam with the charge density 100 nC/cm, which is two orders of magnitude larger than that of the directly coupled SPPs. The tuning band of the mediately coupled SPPs reaches up to 9% for the beam energy ranging from 10 keV to 30 keV, while it almost cannot be tuned for the direct coupling. The lifetime of the mediately coupled SPPs, which reaches up to hundreds of femtoseconds, is also much longer. Accordingly, the mediated coupling may bring great significances for the applications of SPPs.