In this paper, the output current characteristics within a large range of incident light power of a double heterojunction uni-travelling-carrier phototransistor (UTC-DHPT) were analyzed in detail and compared with a single heterojunction phototransistor (SHPT) simultaneously. Firstly, the output current of UTC-DHPT is smaller than that of SHPT under small power incident light because the UTC-DHPT’s absorption section is only the base region, which is smaller than SHPT’s absorption section consisting of base and collector. Secondly, benefiting from the double hetero-junction, the photogenerated carriers are only generated in base in UTC-DHPT, and there are few photogenerated holes accumulated in the collector junction interface. The space charge effect, which is seriously in SHPT, is effectively alleviated. Furthermore, the saturation of the output current under small power incident light, which always occurs in SHPT, is avoided. Therefore, UTC-DHPT has a larger quasi linear range than SHPT. Lastly, the uni-travelling transportation in UTC-DHPT make photogenerated holes in the base region reach to the emitter junction interface by the rapid relaxation, and thereby effectively reduce the emitter junction barrier. This would increase the number of electrons transmitting from the emitter to the base in the unit-time, and the emitter junction injection efficiency is improved. Thus, UTC-DHPT can obtain higher output current than SHPT under the high power incident light.