Surface segregation is a significant phenomenon due to its influence on many surface processes, such as corrosion, oxidation and catalysis. Defects and vacancies produced by ion irradiation in alloys used in reactors or other radiation environments may also induce surface segregation. In this work, we deposit AuCu3 film on a Si(111) substrate by magnetic sputtering. He+ and Au+ produced by pelletron are used to simulate radiation fields in reactors, and surface segregation induced by ion irradiation is investigated. SRIM software is used to simulate ion range and displacements produced in sample. Rutherford backscattering spectrometry is used to determine concentration changes near the surface of sample before and after irradiation. The results show that two kinds of ion irradiations lead to different surface segregation trends. When irradiated by 2 MeV He+, Au elements are segregated at the surface of sample. Oppositely, when irradiated by 1 MeV Au+, Cu elements are observed at the surface of sample. After analysis and discussion, we consider that this phenomenon is induced by different vacancy distributions by He+ and Au+ irradiation. 2 MeV He+ produced Au and Cu vacancies are distributed in whole film from surface to substrate smoothly, except very near the surface the concentration of vacancies has an obvious reduction. As a result, a gradient of the vacancy concentration is formed between the surface and the interior of the film. As the concentration of vacancies on the surface is lower than in interior, it would lead to vacancy diffusion from interior to surface, equivalent to diffusions of Cu and Au atoms along the opposite directions. Because of lighter atomic mass, Cu atom has a faster diffusion rate than Au atom. As a result, the concentration of Au atoms near the surface increases. Unlike He+, Au+ produces a mass of vacancies near the surface of the film, consistent with the Bragg peak by energy deposition of Au+, but decreases rapidly inside the film. It leads to a gradient of the vacancy concentration from surface to interior of the film. When vacancies diffuse from surface to interior, Cu and Au atoms diffuse from interior to surface, the lighter Cu atom concentration increases faster than Au atom concentration. Our research results explain the different segregation trends by light ion with higher energy and heavy ion with lower energy. It may help to understand the surface segregation of alloys used in complex irradiation field.