A direct measurement technique was developed to detect gaseous alkali metals in this work. It was based on the surface ionization principle. To verify the mechanism, a detector probe was designed and calibrated by NaCl samples. The calibration gives a linear correlation between the detected voltage signals and the vaporizing rates of sodium. Then, the calibrated detector probe was applied to detect the gaseous sodium content in flue gas for the prepared Zhundong coal samples during the heating process. These coal samples contained varied contents of sodium, which were classified as raw coal, washed coal, and coal with sodium additives. Results show that sodium compounds in the raw coal vaporize in the temperature range of 300-600 degrees C. Its highest release rate appears at 457 degrees C. For the coal samples with additives of NaCl or Na2SO4, higher addition ratio leads to more released sodium. Compared with SO42-, Cl- has a greater influence on sodium release. This technique may be applied to online measure the gaseous alkali metal content during high-alkali coal combustion in a boiler.