Aiming at the characteristics of oil field sewage, the electrocoagualation (EC) process was used for the experimental study on preparation of oily waste water. The effects of electrode distance, initial pH and NaCl content on oil removal rate were analyzed, respectively, and the influence parameters were obtained. The flow field experiment was firstly carried out . Then, the solution was divided into representative of three layers in the vertical direction, while the changes of oil removal rate under different current density with time were examined, and the characteristic curves of the three layers with the oil removal rate were compared. The results showed that oil removal rate of A layer was the fastest and best, while the oil removal effect of B layer was worse than that of C layer. When electrolysis time was 4 min and current density was 150 A/m2, the oil removal effect of A layer accounted for 96.05% of total efficiency. Considering the average cost and oil removal rate of three layers, the overall treatment was the best in EC reactor when electrolysis time was 16 min and current density was 100 A/m2.