The invention provides a cylindrical Langmuir probe, a probe assembly and a plume flow field detection system, and relates to the technical field of plasma detection. The cylindrical Langmuir probe comprises a tungsten rod needle body, a sheath cylinder and a hollow locking rod; the sheath cylinder and the hollow locking rod are both made from insulated materials; a needle body connecting column is arranged at one end of the tungsten rod needle body; the tungsten rod needle body penetrates out of a first end of the sheath cylinder, and the sheath cylinder is internally provided with a needle body locating cavity which cooperates with the needle body connecting column; a cable through hole for a cable to penetrate through is formed in the hollow locking rod; the hollow locking rod is inserted into a second end of the sheath cylinder, and can push the needle body connecting column to abut against the end face of the needle body locating cavity; the hollow locking rod is locked with the sheath cylinder. By means of the cylindrical Langmuir probe, the technical problems in the prior art that a measuring result of a Langmuir probe is easily interfered with, and the measuring precision is low are solved.