The invention discloses a microwave fluid medium detector. The microwave fluid medium detector is characterized by comprising a microwave transmission line, wherein the microwave transmission line comprises an open-middle-type microwave transmission line and a two-sealed-side-type microwave transmission line; the open-type microwave transmission line comprises a first inner conductor and a first outer conductor; the sealed-type microwave transmission line comprises second inner conductors, second outer conductors and a low-loss microwave medium; the second inner conductors at two sides and the first inner conductor in the middle are connected in a linear mode; the second outer conductors at two sides and the first outer conductor in the middle are integrally connected and mutually form water flow through holes; openings in two ends of the water flow through holes are connected with pipelines through which the to-be-detected fluid medium flows by flanges; the fluid flowing direction of the water flow through holes is mutually perpendicular to the axial direction of the first inner conductor; coaxial wire ports formed by end parts of the second outer conductors and the second inner conductors are connected with a microwave adapter; the microwave adapter is connected with an external microwave measurement device. The microwave fluid medium detector has the advantages that structure is simple, operation is convenient, and the fluid medium can be measured continuously online.