The invention relates to the field of side slope engineering stability evaluation and landslide hazard prediction and prevention, in particular to a downslide thrust evaluation method for variable width slicing of a side slope, and mainly solves the problems that the conventional circular slice method and the residual downslide thrust method have difficulty in meeting the accuracy requirement, and misjudgment occurs easily. The downslide thrust evaluation method comprises the following steps: firstly, comprehensively determining a major slipping longitudinal profile map of a side slope body through exploration and determination of an underlying slipping surface, and according to changes of the inclination theta of the underlying slipping surface, determining the extreme point and the inflection point of the inclination changing curve, so as to obtain sliced regions of the underlying slipping surface of the side slope body by division; performing uniform angular slicing on the sliced regions and determining the slicing width of soil stripes of the overall side slope body, the residual downslide thrust direction and the positions of action points. The downslide thrust evaluation method for variable width slicing of the side slope, provided by the invention, effectively solves the problems of precise calculation and stability evaluation of the residual downslide thrust of a side slope when the underlying bedrock surface is a continuous slowly-varying arc-shaped slipping surface, thereby being extremely high in practicability and effectiveness.