Abstract: In order to improve the transient response speed of fast reactor control system and reduce the overshoot and steady-state error of the controller, a linear active disturbance rejection controller for the coolant outlet temperature of fast reactor was designed. Based on the point kinetics equation, fast reactor reactivity equation and thermal hydraulics model of the reactor core, the second-order equation with total disturbance applied to the design of linear active disturbance rejection controller was derived. Using the parameters of the second-order equation, some parameters of the linear extended state observer (LESO) with model information were determined. Finally, the controller model was simulated in MATLAB environment. The simulation results show that the designed controller has a good performance. Compared with the controller taking the coolant flow variation as the input, the controller taking coolant flow rate as the input has faster response speed, smaller overshoot and better disturbance rejection ability.