The extend state observer(ESO) in the active disturbance rejection control method is studied. The analysis in the frequency domain shows that ESO is effective in the low frequency domain but invalid in the high frequency domain. Two mainly factors, the sampling time and the coefficients of ESO, are discussed, by which a better ESO is obtained. An improved ESO is proposed, which has better performance than the present ESO under same limitations. Simulation results show that the improved ESO is a better observer for active control.