The invention discloses a high-temperature flue gas scaling online monitoring device and process, and belongs to the field of high-temperature flue gas scaling online monitoring equipment. The device is characterized by comprising a flue gas inlet connecting pipe (1), a gate valve (3), a vortex acceleration mechanism, a flue gas outlet connecting pipe (11), a cooling mechanism and a metering device; the inner diameter of the gas inlet end of a pipe body of a variable-diameter casing pipe (5) is larger than that of the gas outlet end, the gas outlet end is connected with the cooling mechanism and the metering device through the flue gas outlet connecting pipe (11), and heat insulation layers (2) are arranged outside the flue gas inlet connecting pipe (1), the gate valve (3) and the variable-diameter casing pipe (5). A suddenly-shrunken variable-cross-section structure is adopted at the flue connecting pipes, the flow line of high-temperature flue gas changes at the suddenly-shrunken part of the cross section, a vortex zone is formed due to diameter variation of the variable-diameter casing pipe, a fixed depositing and scaling zone is artificially formed at the fixed position of the variable-diameter casing pipe, and the dust concentration of high-temperature flue gas in a flue in unit time and the scaling condition of dust in corresponding time can be more conveniently observed in a centralized mode.