In this study, black phosphorus quantum dots (BPQDs) are synthesized by improved ultrasonic-assisted liquid exfoliation. The as-prepared BPQDs are deposited on large-area conductive substrates by electrophoretic deposition, and exhibit high-sensitivity humidity sensing and excellent electrical properties. The results offer not only a facile and efficient method to synthesise stable BPQDs films, but also a new opportunity to develop humidity sensors and nano-electronic devices.