Phototransistor is an important part of photo-electricity encoder circuit for spacecraft, which is sensitive to the displacement damage caused by high energy proton. Relationship between output current which is a key parameter for phototransistor and proton energy, displacement damage dose, working state and shielding materials was studied by irradiation test in this paper. With displacement damage dose increasing by 50, 60, 70, 92 MeV proton, the output current of phototransistor decreased almost 80%. It was caused by the initial photocurrent decreased in photodiode and the gain decreased in transistor. Stainless and sandwich shielding structure have almost no shielding effect on 60 MeV energy protons. The effect of displacement damage can be reduced by increasing the initial photocurrent or optimizing the photodiode to PIN type diode which can increase the depletion area.