A planar conjugated copolymer named HXS-1 was applied in thin film phototransistors. Similar to organic field-effect transistors using VG to control the source-drain current, in phototransistors, the light is used to substitute VG as an independent variable to control the output of the transistors to realize light detection and signal magnification in a single organic device. All devices exhibited high performance with an on/off ratio up to 4.6 x 10(4) (the highest on/off ratio of organic or polymer phototransistors), which could be assigned to i) the wide absorption features of HXS-1 in the whole UV-vis range, ii) the ideal HOMO energy level of HXS-1 (5.21 eV) to align with Au electrodes (5.2 eV) and iii) the high mobility of the polymer thin films (similar to 0.06cm(2)/Vs). Moreover, the devices, both under continuous operation conditions and long term measurement conditions, exhibited excellent stability, indicating potential applications of the devices in polymer-based optoelectronics.