For a bistatic HF radar, the bistatic angle over a clutter cell is a variable. This will cause the spread of the Bragg lines. A new analytical method is proposed to investigate the effect of the bistatic angle on this kind of spectrum spread: both the receive beam width and range resolution are taken into account together, then the Bragg lines width is calculated. The result shows that this spectrum spread is closely related to the clutter cell's spatial position, and the relationship between them is presented. The measured data of some experiments are analyzed and the results demonstrate that this analysis is reasonable.