The flying-wing aircraft has attracted extensive attention at home and abroad in recent years due to the advantages of high aerodynamic efficiency, long endurance time and excellent stealth performance. However, low-order elastic modes of the aircraft tend to be coupled with short-period rigid body modes under aerodynamic excitation, which induces a new flutter morphology - body freedom flutter. The re-search on BFF active suppression in the international aviation field has just started, and the design of MIMO BFF control law is rarely published. In this paper, a novel MIMO control law design method for suppressing BFF is proposed. This control law takes the uncertain factors of the controlled plant as "un-known perturbation", estimates and compensates the "unknown perturbation" through the input/output relationship of the plant, and finally synthesizes the MIMO feedback control law with high robustness. In order to validate the effectiveness of proposed control law in the aspect of the suppressing BFF, a high aspect ratio flying-wing unmanned aircraft is chosen as the research object in this paper, and the MIMO active disturbance rejection controller is designed with the fuselage elevator and the outboard wing flap as the control input, the aircraft rigid body pitch rate and wingtip acceleration as the feedback signal. The numerical simulations of closed-loop root locus, time-domain simulation and minimum sin-gular value analysis were developed, and the results indicate that the presented MIMO active disturb-ance rejection control law design method could improve the BFF critical velocity effectively, specifical-ly, the BFF critical velocity of the flying-wing drone can be increases by about 45.3% with high robust-ness.