Phototransistors are three-terminal photodetectors which usually have higher photosensitivity than photodiodes due to the presence of gate electrode. In this report, organic phototransistors (OPTs) based on a donor material, namely, poly{2,5-selenophene-alt-2,8-(4,10-bis(2-hexyldecyl))thieno[2,3:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione} (PSeTPTI), are fabricated and intensively studied. As unipolar p-type organic semiconductor usually has plenty of electron traps in the bulk to impede electron transporting, most of photogenerated electrons will fill the traps in PSeTPTI and this process can prolong the response time. By introducing [6,6]-phenyl C-61 butyric acid methyl ester on top, the p-n heterojunction can produce most of the photocurrent and eliminates the influence from the process of trapping electrons. This mechanism improves the photoresponsivity and response speed. Since ultraviolet (UV) detection is very important in some fields including military, aerospace, and biology, the OPTs are characterized under UV illumination besides the visible light and they present high sensitivity. Furthermore, organic semiconductors often have bad stability in harsh conditions and meanwhile some devices need to work in these environments. At high temperature even up to 200 degrees C, our OPTs can work normally and show very high stability, indicating the potential of the devices in applications of high-temperature environments.