The Fisher information matrix was adopted to identify the parameters of a lithium-ion battery, and an online SOC(state of charge) estimation method was proposed based on the SP2D (simple pseudo-two-dimensional) model. Compared with the results of the equivalent circuit model, the experiment verifies that the proposed SOC estimation method has higher precision. Effectively solving the problem of SOC high accuracy estimation in the whole range of SOC, the average error of SOC can be reduced by nearly 30%, and by 60% in the later stage of battery discharge.