The invention provides an in-situ flue gas online measuring device which mainly comprises a sampling probe, a filter, a measuring cavity, a quartz tube, a transmission window, a cube corner reflector, a power pumping device, a laser transmitting and receiving module and an analyzer. The in-situ flue gas online measuring device is mounted inside a flue. Flue gas can flow through the filter and then flows into the measuring cavity by the aid of negative pressures of the flue under the effect of the power pumping device and then flows back to the flue after the flue gas is completely measured. The in-situ flue gas online measuring device has the advantages that the in-situ flue gas online measuring device is mounted inside the flue, the temperatures of measurement environments and the temperature of the flue gas constantly keep consistent with one another, the quartz tube is mounted inside the measuring cavity, accordingly, components in the flue gas can be guaranteed against being changed when the flue gas flows into the measuring cavity, and particularly, adsorptive components (such as ammonia gas) in the flue gas can be accurately measured; the flue gas can be pumped by the aid of the negative pressures of the flue, accordingly, air does not need to be compressed by the aid of instruments, and energy can be saved; the measured flue gas comes from the flue and ultimately flows back into the flue without flowing into atmosphere, and accordingly environments can be protected.