Determining a white plume control line of smoke after wet desulphurization includes: drawing a saturated air enthalpy humidity curve or equivalent; obtaining annual temperature and humidity change data of located cities or regions along with the time at the frequency of at least one datum every day; drawing the data obtained in the saturated air enthalpy humidity curve; and drawing a tangent line on the saturation curve by using each meteorological point in a chart, the right lower side of the tangent line is a de-pluming control region, the de-pluming effect superior to that at the feature meteorological point can be realized when the smoke enters the region after regulation, a region defined by the de-pluming control line and the saturation curve at a low-temperature side forms a de-pluming day number control region, and the point number falling within the region is the white plume generating day number.