In order to improve the luminous efficiency of conventional GaN-LED, a transmissive surface plasmon enhanced GaN-LED based on nano-grating was proposed. The enhanced LED included a low refractive index SiO2 film and a Ag film in the p-GaN grating slot, and the top layer ITO film. The basic principle of improving LED emission characteristics for this new structure was described in detail, and with simulation software "COMSOLTM-RF Module" based on the finite element method, the parameter optimization and numerical simulation analysis for the structure were studied. The results show that when the period p=280 nm, the duty ratio f=0.5, d■=25 nm, dAg=15 nm, dITO=30 nm, the transmission efficiency of the structure is higher in the visible range, and the 0th order transmittance is 0.716, the 0th order reflectivity is 0.224, the peak of the -1st order transmittance is 0.183, with Purcell factor enhancing nearly 16.4 times. The internal quantum efficiency, light extraction efficiency and SPP extraction efficiency can be improved simultaneously in this new GaN-LED.