In order to reduce the computational complexity of the electrochemical modelling of lithium-ion batteries and improve the computational speed, this paper proposes a simplification method for the modelling of lithium-ion batteries. Based on the existed single particle model of lithium-ion battery, this article simplifies the solid-liquid diffusion process and proposes an extended single particle model. Simulation based on COMSOL is performed to verify the proposed model. Compared with the full order electrochemical model, the calculation speed of the proposed method under 1C discharge rate only takes 0.6054 seconds, which is only 10.9% of P2D. In addition, the mean square error under constant current condition is 2.95 mV only, and the dynamic condition error is 10.12 mV. The results verify the superiority of the proposed model in calculation speed with enough accuracy.