The invention relates to the field of detection of flue gas from a coal-fired boiler, in particular to a flue gas sampling device. The sampling device is used in an on-line flue gas monitoring systemso as to monitor the flue gas pollution and emission situations of a power plant boiler. In order to solve the problem that in an existing flue gas sampling device, the deviation of sampled flue gas sources is large so that the sampled flue gas cannot represent average pollutant concentration, the flue gas sampling device comprises a sampling pipe in an inner cavity of a gas flue, one end of the sampling pipe is connected with a suction device and a gas inlet pipe of the on-line flue gas monitoring system, a plurality of sampling nozzles with openings are arranged on the sampling pipe in the length direction of the sampling pipe, the sampling nozzles are communicated with the sampling pipe, and the section of the opening in each sampling nozzle is gradually increased from one end of the suction device to the other end of the suction device. In the flue gas sampling device, the sizes of the openings of the sampling nozzles are changed, the adverse effects, caused by long distance and weak suction force, on the suction volume of the flue gas are improved through the pore diameter trend change of the openings, and the uniformity that all the sampling nozzles suck the flue gas is improved, so that the representativeness of a mixed flue gas sample which is finally collected is enhanced.