An approximate method for quantifying the observation error of the linear extended state observer(LESO) is proposed. By linearizing “total disturbance”, the analytic expression of the observation error of the LESO is derived in the time domain, including the dynamic response part and the steady-state static part. The analytic expression of the static error is then proposed as the quantitative expression of the observation error. This method is used to discuss the effect of different construction methods on the observation accuracy of the observer, and the fault tolerance of the observer is analyzed in the case of inaccurate modeling and input deviation. Simulation results verify the correctness and practicability of the conclusions obtained, which indirectly proves the feasibility of using this quantified expression as an approximate method to describe the observation precision of the LESO.