Electrochemical modeling is of great significance for estimating the operational state, performing full-life-cycle fault diagnosis, and enabling multi-condition safety management of lithium-ion power batteries. However, early proposed pseudo-two-dimensional electrochemical models are difficult to apply in engineering practice due to issues such as numerous parameters and challenging identification. To address the problem that current LSP2D models based on approximate reconstruction of solid–liquid phase diffusion processes struggle to meet high-rate conditions, this paper proposes a reduced-order reconstruction method for lithium-ion battery electrochemical models based on an improved LSP2D model. By refining the third-order parabolic approximation of the liquid-phase lithium-ion concentration and innovatively introducing a liquid-phase diffusion characterization term during the approximation process, the method achieves accurate characterization of the lithium-ion diffusion process dominated by concentration gradients under high-rate conditions. Simulation results show that while computational efficiency remains comparable, the improved LSP2D model outperforms the traditional LSP2D model in prediction accuracy across various rates. For liquid-phase lithium-ion concentration, the prediction accuracy of the improved LSP2D model is increased by over 86.96% compared to the traditional model at discharge rates of 1C–7C. For terminal voltage, the improvement exceeds 97.12% at 1C–3C discharge rates, and remains above 29.56% at 4C–7C discharge rates. Furthermore, the theoretical reasons for the significantly lower accuracy improvement at 4C–7C compared to 1C–3C are discussed, providing direction for subsequent research on reduced-order reconstruction of electrochemical models under high-rate conditions. The proposed method offers a new approach for high-accuracy reduced-order reconstruction of lithium-ion battery electrochemical models and contributes to enhancing the engineering practicality of electrochemical models for lithium-ion power batteries.