In order to reduce safety risks related to the field personnel and address the difficulties for deploying in-situ sensors, a novel ultralight wet chemical sensor was developed for the in-situ simultaneous determination of both phosphate and nitrite. The sensor only weighted 220 g and consumed 0.4 W average power, and achieved limit of detections (LODs) of 0.38 and 0.12 μmol/L for phosphate and nitrite, respectively. Moreover, a small experimental multi-rotor unmanned aerial vehicle (UAV) was also fabricated for employing the sensor for the in-situ detection in surface waters.A dual-wavelength and dual-beam detection method that required fewer sensor parts was developed for the simultaneous detection of phosphate and nitrite. Bubbles cut from bubble wrap were used to store liquid reagents. A tandem sampling-reaction-detection cell that enabled merely by rubber bands was designed for avoiding the use of heavy pumps and valves. With these features, the complexity, size, weight and power consumption of the sensor were impressively reduced, hence rendering the sensor especially adaptive to the precious payload capability and battery power of the UAV.To demonstrate the practicability, the developed sensor-UAV system was successfully applied in two water bodies. The UAV flew to the designed water area, then landed and floated on the water surface. The sensor was immersed into the water for in-situ detection, and previously established calibration curves were used for the quantification of the target analytes. The results were compatible with those obtained by the standard methods.