Abstract: There is a "hook-shaped" echo from sea or land in HF backscatter system, which interferes with ship detection of HF skywave radar. In order to simulate the phenomenon, time-varying ionospheric model is built, and the Doppler shift of ray is analyzed to get Doppler information. The simulation results show that the phenomenon is mainly caused by the ionospheric propagation on low and high angle, and the changing of electron concentration of ionosphere. The Doppler and range of the"hook" is different when ionospheric parameters change.