Abstract: In order to resolve the problem that the load's swing angle and swing angle-velocity are hard to measure directly for overhead crane or gantry crane in engineering practice,the state observer is designed by using trolley's position information,and the load's swing angle and angle-velocity are estimated.The difference value between the state observer's output and the trolley-load system's output is introduced and adjusted by the observer's gain vector,then it is sent to the observer's input terminal.The observer's poles are placed at the negative real axis on complex plane,and the system can estimate the swing angle and swing angle-velocity stably and quickly.Simulation results show that the obtaining method of load's swing angle information based on state estimation is practicable and the observer has strong robust property and can be more widely adaptive for the changes of load mass and rope length when the poles increase.However,estimation error will be up or down rushing when the poles are too large.Observer is more sensitive to the change of rope length than the change of load mass when the poles are determined.