An Active Disturbance Rejection Controller (ADRC) can suppress uncertain disturbances effectively; however, its parameter tuning is difficult. In order to develop an adaptive linear ADRC, the parameter tuning strategy of a linear ADRC was studied. First, an adaptive tuning algorithm based on observational errors was designed for a linear extended state observer. Then, an adaptive tuning algorithm for the parameters of the ADRC linear feedback component was designed. Finally, by using the Lyapunov method, it was proved that the parameters obtained by the above adaptive tuning algorithms ensure that both the observation errors of the extended state observer and the output errors of the controlled system converge to zero. Experimental results indicate that, under low velocity conditions, the parameters of the proposed linear ADRC can be quickly tuned within 0.8 s; the observational errors of the linear extended state observer are less than 2 nm, and the velocity fluctuations of the precision air-bearing platform are within 5%. The proposed adaptive linear ADRC performs online tuning of the controller parameters, and the controller performance is satisfactory.