Abstract: In traditional absolute nodal coordinate formulation (ANCF) modelling method, the neutral line of the beam with variable cross-section is assumed consistent with structural median line. It will reduce predicting accuracy in flexible deformation and dynamic behaviour for the beam with variable cross-section. For solving this kind of problems, a new dynamic predicting method for slender beam with variable section is presented in this paper. The neutral line position errors of beam with variable cross-section caused by variable section structure and composite material properties are considered to establish the displacement field in this method. Combining with the strain feedback control law and the proposed method, the active vibration control strategies for this beam under space thermal load are proposed by using the sensory and actional piezoelectric patches. Meanwhile, the correctness and accuracy of the proposed method has been verified by taking the variable section solar-sail masts into numerical analysis with the proposed method. According to the numerical results, the higher prediction accuracy can be provided by the proposed method, and the flexible deformations of solar-sail mast with variable section caused by thermal load have been effectively suppressed by the strain feedback control law. For obtaining the optimized control results in flexible deformation of the beam with variable cross-section, the dynamic behaviour of it with different piezoelectric ply control strategies has been further analysed. According to the numerical results, the active vibration control will be deteriorated with the assembling distance between sensing and actuating piezoelectric plates increasing. In addition, the stability and sensitivity of the control system can be improved by raising the controller gain. However, the unsteady vibration of the beam will also be excited with the overshoot controller gain. The theoretical value and practical significance in the compositive technical performance can be provided for the beam with variable cross-section during on-orbit working process.