There is a problem of power and efficiency fluctuations caused by coil offset in a wireless power transmission (WPT) system. Thus this paper proposes an improved model of the WPT system based on bilateral LCC (DS-LCC) topology. First, it analyzes the transmission characteristics of the model, and derives the corresponding functional formulae between the coil mutual inductance and the transmission characteristics under different lateral offset conditions. Secondly, a normalization method is introduced and the linear relationship between the number of coil turns and the coupling strength and the internal resistance of the coil after the offset is determined. By optimizing the design of the system parameters, the system output power and transmission efficiency anti-offset within a specific lateral offset range are improved. Finally, a 100 W experimental prototype is built to verify the theoretical analysis. The results show that the output power of the system is always higher than 80 W within the lateral offset range of 0-20 cm, and the transmission efficiency is always higher than 70%. This work is supported by Shanghai Natural Science Foundation Project (No. 21ZR1425300).