Monitoring P flux at the Earth's surface-atmosphere interface has many challenges. Therefore, the development of a technology with high selectivity and high sensitivity to in situ trace PH3 in aquatic or sedimentary environments has become a priority. Herein, an amperometric PH3 microsensor meeting the above conditions is developed. The sensor is equipped with a Au-coated Pt working electrode (WE) and a Pt guard electrode (GE) positioned in an outer glass casing. The WE and GE are polarized at a fixed value of +150 mV with respect to a pseudo-reference electrode. The outer casing is filled with an acid electrolyte solution, and the tip is sealed using a thin silicone membrane. Mixed gases from the environment diffuse through the first layer of the silicone membrane, and the major H2S disruptor is eliminated by a ZnCl2-propylene carbonate trap positioned in the front of the microsensor. Later, the gases diffuse into an electrolytic solution through the second layer of the silicone membrane, and PH3 is selectively oxidized into H3PO4 on the Au-coated Pt WE. This electrochemical oxidation thereby creates a current that is proportional to the concentration of PH3 (>2 nmol·L-1). With the aid of the H2S trap casing and selective catalysis, the effects of other gases on the microsensor can be ignored in terms of environmental monitoring. An example from the sedimentary profile shows that high PH3 accumulations are found 13 mm below the sediment surface.