The usual mode sets for the dynamics modeling of flexible spacecraft are reviewed. Some mode reduction criterions for the selection of system retained modes are summarized. It is pointed out that the modal cost analysis and the internal balance theory are for the system mode selection and there is the problem how to use these criterions for the component elastic mode or the system elastic deformation mode selection. A tactic for using the order reduction criterions for the dynamics modeling is introduced. An example is given to show the comparison of modeling results with different mode sets and to illustrate how to use the mode reduction criterions for the dynamics modeling of flexible spacecraft.