Abstract: Electron beam rapid prototyping machine has high working frequency, trajectory deviation can be caused by many factors, such as iron losses and eddy. We conducted interpolation algorithm to restrain dynamic deviation by getting deflection scanning parameters. However, the way to acquire parameters through optical observation system has relatively low precision. In order to improve the trajectory precision, a parameter acquisition device is installed on the existing electron beam rapid prototyping machine, it can collect secondary reflection electrons and convert them into voltage signal and make it displayed. We can judge the relative position of hole center and electron beam center by the displayed signal. Experiments show that: when the electron beam focuses on the upper surface and its spot locates in the center of the feature point, secondary reflection electron signal is the minimum; meanwhile, the recorded deflection and scanning parameter has high accuracy, therefore the deflection scanning trajectory can be greatly improved.