To address the machining deformation of aero-engine accessory casings, a real-time monitoring and control method based on a floating fixture system was proposed. An experimental part with typical thin-walled deep-cavity features of an accessory casing was constructed, and a pneumatically actuated floating fixture system consisting of four clamping cylinders and six supporting cylinders was developed. A dynamic loading–unloading strategy was embedded in the supporting cylinders to achieve staged release and redistribution of concentrated machining stresses. Piezoelectric and laser sensors were integrated in the supporting region to obtain force and displacement histories at weak-stiffness locations before and after stress release. Using a finite element simulation model, the measured axial supporting force was converted into machining stress and deformation fields. The results show that, compared with conventional fixed support, the dynamic floating support reduce the average stress by 72.16% and decreased the deformation by 67.11%. These findings demonstrate that the proposed floating fixture system and control strategy effectively suppress residual stress accumulation and improve the geometric accuracy of thin-walled deep-cavity accessory-casing components.