The invention discloses a GeSn-GeSi material based heterogeneous phototransistor and a fabrication method thereof. A collector and an emitter of the transistor both adopt a GeSi material, a light absorption region and a base region both adopt a GeSn material, an emitter region, the base region, the light absorption region and a collector region are sequentially and vertically arranged, and a passivation layer encircles the peripheries of the emitter region, the base region, the light absorption region and the collector region. According to the fabrication method of the transistor, the GeSn material is grown by a low-temperature solid-source molecular beam epitaxial process, and the fabrication method is a standard complementary metal oxide semiconductor (CMOS) fabrication method. The GeSn material with a high light absorption coefficient forms heterojunctions in the light absorption region, the GeSi emitter region and the collector region, the light sensitivity and light current during detection of an infrared light signal by the transistor are improved, and the GeSn-GeSi material based heterogeneous phototransistor has high light absorption rate.