NO2 sensors with ultrahigh sensitivity are demanded for future electronic sensing systems.However,traditional sensors are considerably limited by the relative low sensitivity,high cost and complicated process.Here,we report a simply and reliable flexible NO2 sensor based on single-layer MoS2.The flexible sensor exhibits high sensitivity to NO2 gas due to ultra-large specific surface area and the nature of two-dimensional (2D) semiconductor.When the NO2 is 400 ppb (parts per billion),compared with the dark and strain-free conditions,the sensitivity of the single-layer sensor is enhanced to 671% with a 625 nm red light-emitting diode (LED) illumination of 4 mW/cm2 power under 0.67% tensile strain.More important,the response time is dramatically reduced to ~16 s and it only needs ~65 s to complete 90% recovery.A theoretical model is proposed to discuss the microscopic mechanisms.We find that the remarkable sensing characteristics are the result of coupling among piezoelectricity,photoelectricity and adsorption-desorption induced charges transfer in the single-layer MoS2 Schottky junction based device.Our work opens up the way to further enhancements in the sensitivity of gas sensor based on single-layer MoS2 by introducing photogating and piezo-phototronic effects in mesoscopic systems.