In this paper, hetero-junction structure of composite film based on α-6T/ PC61BM was firstly utilized as active layer of OTFT gas sensor for detecting ammonia gas (NH3). The electrical characteristics, gas sensing performances based on α-6T/PC61BM composite film and pure α-6T film were tested and compared. It could be found that the sensitivity of α-6T/PC61BM composite film was much higher than pure α-6T film, increasing from 1.02 × 10−3 ppm−1 to 3.63 × 10−3 ppm−1. In addition, the sensing mechanisms of all sensors were studied as well.