The invention discloses a method for treating mercury-containing wastewater during PVC (Polyvinyle Chloride) production through a two-step process. The method comprises the following steps of: firstly, completing primary removal of mercury, copper, ferrous and ferric iron, cadmium, zinc, manganese, lead and suspending impurities of above 10mg/l by using a precipitator and a flocculating agent, regulating PH to 6-8 and then adding the flocculating agent and stirring for 30 minutes, standing for above 1 hour, discharging slag and aerating, absorbing mercury carried away by a gas phase through a sulfide adsorber, purifying water by using a plate type kieselguhr filter, removing residual suspended matters; and 2, carrying out complete reaction on the mercury in the wastewater by using an active carbon and a mercaptan and thiourea resin combining method again for being removed, and finally reaching the standard that the content of the mercury is less than 0.005mg/l. The treated wastewater can be recycled to salt melting or an acetylene generator, so that zero emission of the treated wastewater is achieved; and the mercury-containing waste acid water of hydrochloric acid desorption in the industry of chlor-alkali can be treated, the consumption of acid is reduced, and the great popularization significance is achieved in the industry of chlor-alkali.