A general linear active disturbance rejection control(LADRC) approach is proposed for a class of single-input single-output(SISO) high-order nonlinear systems with subject to dynamical and external uncertainties. For every part of LADRC, the principle of high-gain observer with single parameter tuning is adopted to construct the linear tracking differentiator, the linear extended state observer and the linear state error feedback law. By using Lagrange mean value theorem and Cauchy-Schwarz inequality, the differential value of the system's total disturbance is transformed into a function with respect to the system estimation and tracking errors, thus solving the problem that the differential value of control input is difficult to be determined in advance due to the unknown control gain. Then it is proved that the errors of the closed-loop system are bounded based on the Lyapunov stability theorem. By further analysis, the quantitative relationship between the estimation and tracking errors with the control parameters is derived, and both can be infinitely small as the observer gain increases. Numerical simulation results show the effectiveness of the proposed approach. Compared with Han's ADRC, the proposed LADRC has simple structure, few tuning parameters and easy implementation in engineering.