The invention discloses an on-line measurement device for gaseous-phase alkali metals in a coal-fired flue gas. The on-line measurement device comprises a fume sampling main tube, at least one fume sampling branch tube, ionization devices, signal acquisition devices and a power supply, wherein the gas inlet of the fume sampling main tube is communicated with an external flue to be detected; the gas outlet of the fume sampling main tube is communicated with gas inlets of the fume sampling branch tubes simultaneously; each fume sampling branch tube is correspondingly equipped with one set of the ionization devices and one set of the signal acquisition devices; the ionization devices comprise ionization chambers, charge generators and charge sensors; the gas inlets of the ionization chambers are communicated with gas outlet pipelines of the fume sampling branch tubes; the charge generators and the charge sensors are both coaxially mounted at gas inlets of the ionization chambers; the signal acquisition devices comprise signal amplifiers for converting current signals into voltage signals and signal acquirers for acquiring the voltage signals. The on-line measurement device has the beneficial effects of being simple and portable and applicable to on-site boiler measurement environments, and alkali metals, particularly contents of Na and K, in fume can be measured on line.