The produced water of SHM gas field was characterized by high oil content， high suspended solid content， high salinity， weak acid and slow separation of oil and water， which affected the normal operation of re-injection system. The produced water was treated by coagulating sedimentation， and the types and dosages of pH regulator， iron removal agent， inorganic flocculant and organic flocculant were studied. Finally， the effect of dosing interval and dosing order on flocculation was researched. The results showed that the water quality changed from weak acid to neutral after the following steps. First， 0.5 ml/L of iron removal agent hydrogen peroxide solution was added to the wastewater. Second， the pH value was adjusted to about 7.5 with NaOH 3 minutes later. Third， 100 mg/L inorganic flocculant PAC was added 1 minute later， and 6 mg/L organic flocculant cationic polyacrylamide was added 2 minutes later. After the treatments， the complete flocculation time of floc was 8 min， and the suspended solid content decreased from 400—700 to 14—30 mg/L， the oil content decreased from 722—1366 to 32—42 mg/L， which met the quality requirement of injection water.