Abstract: Chlorides in the wastewater with high concentration seriously interrupts the determination of chemical oxygen demand (COD). Microwave digestion method was used to test COD of wastewater with high chlorine. The experiment results show that for this method, the effect of Cl^- is weaker than that by traditional potassium dichromate method. As the concentration of Cl^- is between 0~30 000 mg/L and COD≤500 mg/L, a certain amount of mercuric sulfate would be added during the COD test, and fractional error of test result is less than±5%, which was in the scope of national standard. When the COD of shell gas fracturing flow-back fluid was tested, the effect of Cl^- on the COD test would be eliminated by using microwave digestion method and the test results was accurate and reliable.