The reserves of high alkali coal are abundant in the Zhundong area of Xinjiang, but in the process of its combustion and utilization, the release of alkali metals such as Na and K into the flue gas in the form of gaseous phase will cause ash deposition, slagging, and corrosion, etc. on the heat exchange surface of the boiler. The on-line detection of the alkali metal release in the gas phase has certain guiding significance for solving the above problems. A portable system for detecting the concentration of gaseous alkali metals in the flame was developed based on flame spontaneous emission spectrum. Using this system, the on-line detection of high alkali coal combustion flame in liquid slag removal cyclone boiler was carried out. The flame temperature, gas phase Na concentration and band thermal radiation energy at different measuring points were obtained, and X-Ray Fluorescence Spectrometer (XRF) was used to analyze the ash and liquid slag samples, simultaneously. The results show that the gas phase Na mass concentration in the furnace is unevenly distributed. The average flame temperatures of the two slag chambers burning Hami coal and Shenhua coal are 1 415 and 1 436 ℃, respectively, and the gas phase Na mass concentrations are 40.0 mg/m3 and 25.3 mg/m3, respectively. The gas-phase Na concentration and band thermal radiation energy can reflect the combustion state in the furnace, and the measurement results have a certain correlation with the composition analysis results of fly ash and liquid slag. The Na mass concentration in fly ash and liquid slag increases, but more migrated to the fly ash.