A composite structure of dielectric grating/metal film with silver nanocubes is theoretically designed, and the ultrahigh electric field enhancement factor in the composite structure is numerically simulated using the finite element method. The surface plasmons are excited by the laser with wavelength of 442 nm. By discussing the extinction spectrum of the silver nanocubes with different sizes, and the reflection spectra of the grating with different period and thickness, the optimal parameters are selected, which the side length of silver nanocubes is 70 nm, the period of grating is 312 nm and the thickness is 90 nm. Under the condition of optimal parameters, electric field intensity distribution in the composite structure is calculated. For the resonant coupling of local surface plasmon and propagation surface plasmon in the dielectric grating/metal film with silver nanocubes composite structure. The electric field enhancement factor of hot spot is as high as 1.53×106, which exists in the position between grating ridge and the lower vertexes of silver nanocube. The ultrahigh electric field enhancement generated by the composite structure has the potential to be applied to the study of surface-enhanced Raman scattering.