A structural damage diagnosis method based on residual modal force is proposed for the damage diagnosis of equipment cantilever beam structure. The concepts of residual modal force (residual modal force vector) and structural element damage factor (structural element damage factor vector) are defined, which are applied to judge whether the equipment structure is damaged and indicate the location and degree of the damaged element, and the simulation experiments of 3 states are carried out respectively. The experimental results show that the method can effectively identify the damage type, location and degree of the equipment cantilever beam structure, and is scientific and effective for the equipment cantilever beam structure diagnosis.