The invention relates to a post-desulphurization flue gas moisture content determination method and apparatus. The apparatus comprises a sampling tube, a heat tracing ventilation pipe, a gas suction pump, a flow regulating valve, a flow meter, a three-way valve, a temperature measuring instrument, a pressure measuring instrument, and a Karl Fischer titrator, which are connected sequentially. The determination method comprises the steps that: (1) 100ml of anhydrous methanol is added into a titration cup, and the titrator is reset to a system blank value. (2) standard water is accurately measured, and is injected into the titration cup; and the concentration of a Karl Fischer reagent is measured by using the titrator; (3) the pipeline is washed by using flue gas for 10-15min; (4) a flue gas flow rate is controlled at 50-100mL/min, and a ventilation time is 2-5min; and titration is carried out by using the titrator; and (5) with the titrator, the post-desulphurization flue gas moisture content is automatically calculated. According to the post-desulphurization flue gas moisture content determination apparatus provided by the invention, a Karl Fischer titration method is used in flue gas moisture content determination, the analysis operation is simple and fast, and analysis result is accurate and reliable. Therefore, an instant and accurate basis can be provided for production operation adjustment and data calculation.