In order to improve the light emitting efficiency of GaN-based blue LED, a novel LED model, which has a layer of silver nanodisk array embedded above the quantum well, is designed. The rates of spontaneous emission change and light extraction efficiencies of the models with various geometric parameters are calculated by using finite-difference time-domain method. With the analysis for near field distribution of active region (AR) and far field directivity of LED, we theoretically explain the influence of surface plasmon polaritons (SPP) formed on silver nanodisks on the improvement of LED performance. This model could contribute to the coupling between SPP and AR, leading to the enhancement of spontaneous emission of AR. Besides, due to the compensation for wave vector of SPP by lattice vector of silver nanodisk array, the localized SPP could radiate to space and light extraction efficiency of LED could be improved. Optimization results show that on the condition that the diameter of nanodisk is set to 120 nm and its thickness is chosen as 30 nm, the spontaneous emission is enhanced by 3.6 times compared to that of the conventional LED. Furthermore, as the nanodisks are arranged to the array with triangular lattice and the lattice constant is 220 nm, light extraction efficiency gets 2.5 times enhancement. These results provide a theoretical reference on the practical design and optimization of highly efficient GaN-based LED.