This article considers the robust control problem of non-linear magnetic levitation system with parameter uncertainties and external disturbance. To obtain a simple and practical controller that does not depend on the exact model information, a generalised-proportional-integral observer is constructed to estimate the velocity and disturbance. Then, based on mild assumptions and the estimated values, an output feedback tracking controller is proposed such that the reference position tracking task could be realised. The obtained control approach is confirmed strictly by theoretical analysis. Finally, simulation results are provided to show the effectiveness of the proposed control method.