Abstract: Aiming at the problems of poor adjustment effect and difficult parameter setting of traditional linear active disturbance rejection control（LADRC） for high-order plus time delay（HOPTD） systems, a high-order time delay-linear active disturbance rejection control（HTD-LADRC） was proposed. On the basis of first-order LADRC, a high-order feedforward compensator was connected in series to solve the problem of asynchronicity between feedforward signal and feedback signal of the linear extended state observer（LESO） and the accuracy of state observation of the system was improved. On this basis, the objective approximation legal quantization parameter setting formula was used, and the adjustable interval of parameters was derived to realize single parameter λ adjustment of controller parameters and simplify parameter setting. Furthermore, the frequency domain analysis method was used to verify the effectiveness of the tuning method, and the relationship between the parameters and the system robustness was determined. Finally, in the simulation experiment of selective catalytic reduction（SCR） denitration control system, the proposed controller was compared with other controllers to verify its superiority. Results show that the proposed high-order linear active disturbance rejection control with time delay has obvious advantages in fixed value following, disturbance resistance and robustness, and has great engineering application potential.