In order to solve the problem of external disturbances and system parameter uncertainties in the trajectory tracking of an omni-directional mobile robot, a passivity-based active disturbance rejection control method is proposed. Firstly, the system disturbances are estimated by the extended state observer, then the disturbance compensations are added in an controller to eliminate the influence of external disturbances and parameter uncertainties in the system. By using the passivity theory and Lyapunov theory, it is proved that the closed-loop system is a bounded input, bounded output stable system. The simulation results show that the proposed control method has fast response speed, high control precision and strong robustness to external disturbances and uncertainties of model parameters.