Abstract: Mercury is easily volatile and vaporizes in air. The monitor of mercury in gas phase provides insight for developing advanced Hg control technologies. However, the analytical method of gas mercury is rudimentary. In order to determine the gas mercury content, a reliable calibration method is needed. This study provides a method of calibrating gas mercury from pyrolysis of coal with Hg²⁺ as a precursor. The method is based on the principle that Hg²⁺ in aqueous solution can be quantitatively reduced by reductant into elemental mercury vapor to determine the calibration of mercury content in gas phase. Compared with the conventional calibration method based on the saturated vapor principle, this method eliminates the influence of the sampling temperature, the quantity of mercury during sampling can be easily controlled, and pollution of the gaseous mercury stored in the laboratory is avoided. To illustrate the accuracy of the calibration method, the quantity of elemental mercury emission during pyrolysis of four coals were determined. Results show that the method has good reproducibility and adaptability. Compared with the Canadian Ontario-Hydro method, the relative standard deviations of both methods are less than 3.0%, indicating a high reliability of this calibration method.