Dynamic modeling method of onorbit maneuvering of freefree slender flexible space structures was studied. In literature, a dynamic model without rigidflexible coupling terms was adopted for designing motion controller. Establishing a dynamic model with relatively accuracy is the premise of obtaining a welldesigned motion controller, therefore, it is necessary to establish the coupling model and find the difference between them. A floating reference frame was chosen as local reference frame, whose origin is consistent with the center of mass. The assumed mode method was used for discretizing the deformation parameter. The rigidflexible coupling dynamic equations of structure undergoing large overall plane motion were obtained based on the virtual work principle. Through several simulation examples, the difference between uncoupling equations and coupling equations was discussed, and the results show that the uncoupling model is not accurate enough and simple zeroorder model is more accurate and effective. The analysis provides an important basis for the subsequent motion controller design and vibration attenuation.