Abstract: Automatic placement of prepreg patches based on vacuum fixture is an effective way to achieve high quality and high efficiency of scarfing repair of fiber reinforced plastics (FRP) components. How to reduce the deformation of prepreg patches in the placement process is a prerequisite for effectively controlling the placement defects such as wrinkles and warps. The fundamental experimental equipment is independently developed to lay prepreg patches automatically for scarfing repair. Fundamental experimental researches on adsorption grasping, transferring, and laying prepreg patches by the vacuum fixture are innovatively undertaken. The influence of the temperature, the basic size and the distribution of vacuum suckers and the cantilever length on the deflection variation of the patch are analyzed. The result shows that setting a reasonable laying temperature is essential for ensuring the quality of the laying, and should be 25 ℃. The square suckers should be used for reducing the deflection. For the patch dimension is less than 120 mm×120 mm and the cantilever length is less than 31 mm, the deflection variation caused by self-weight is less than 2.75 mm. It is useful to reduce the degree of defects such as wrinkles and warps which within the tolerance range. Based on the online observation, four simplified mechanical models are established: cantilever beam model in the state of cantilever, simple supported beam model in the process of spreading, cantilever beam model in the state of warp at the corners, and simple supported beam model in the state of arching in the middle. These modes are used to analyze the effect of the position and deformation behavior of prepreg patches during grasping and pressing on the degree of defects such as wrinkles and warps. The fundamental research and processing of placement are theoretically guided by the aforementioned conclusions.