For 3-DOF helicopter system, the problem of fault estimation based on adaptive observer is studied.Firstly, a nonlinear model of a 3-DOF helicopter is established, and a fault estimation method based on proportional adaptive observer is studied.Furthermore, in order to improve the dynamic performance of fault estimation of 3-DOF helicopter system, a proportional-derivative adaptive observer is constructed.The robustness of the proposed fault estimation method is guaranteed by introducing H∞ performance, and the observer gains are calculated by using Linear Matrix Inequalities (LMI).Finally, the designed fault observer is simulated and verified by 3-DOF helicopter simulation system.The results show that, the proportional-derivative adaptive observer has higher accuracy and speed in fault estimation compared with the proportional adaptive observer.