Black phosphorus nanosheets (BP NSs)-decorated single-layer WS2 phototransistors were fabricated by drop-casting the liquid-phase exfoliated BP NSs atop the channel of single-layer WS2 grown by chemical vapor deposition technique. It was found that, upon the decoration of BP NSs, the BP/WS2 hybrid phototransistor exhibited an enhancement of responsivity and extension of response spectral range. The responsivity is up to 120 mA/W in the hybrid phototransistor, being about 20 times larger than that in devices based on the pristine single-layer WS2. Moreover, the hybrid device possess high response speed with the response and recovery time of ~54 and ~140 μs, respectively, and also excellent photoswitching stability.