Micro-eletromechanical system (MEMS) thermal-control shutters for spacecraft are fabricated by using the flexible Al/PI film, because of its light mass, no brittleness and withstanding severe mechanical environment (mechanical environment adaptability) in space. But the stress in the film would be able to bend the shutters too much to fabricate shutter array. Therefore, how to control the thin film stress is an important problem and it is necessary for flat shutters to take some measure to remove or reduce the thin film stress. This internal stress in the thin film formed intricately during the deposition process would make the film exhibit macroscopic compressive stress. So it is difficult to control the thin film stress micro-mechanically, but macro-mechanically. According to the results of the current study, the controlling technology of thin stress is commonly applicable to rigid substrates. In this paper, the flexible Al/PI film may be controlled by interface alloying. We put forward a way of adding Sn layer to the flexible Al/PI film, which makes Al/Sn interface to be alloyed as a measure to control the stress. In the alloy phase, lattice expansion and distortion results in the emergence of transverse shearing stress. The intrinsic compressive stress can be canceled out by the transverse shearing stress and the apparent stress in the films decreases consequently. The Sn atoms diffusion behaviour is proved to form Al-Sn alloying layer by SEM and EDS. This method can be used as a new technology of controlling thin film stress.