In order to accelerate convergence rate and promote observation precision of linear ESO (extended state observer), an improved linear ESO used in active disturbance rejection control system was proposed. The improved linear ESO followed error controlling principle and used the errors between the state variables and their observed values as the regulation basis. It is proved that the observation error of this improved secondorder and third-order linear ESO is stable, and its steady state error is greatly reduced when compared with the traditional linear ESO. Simulation results show that the secondorder and third-order improved linear ESO is faster and more accurate than the traditional same order linear and nonlinear ESO. 