In this paper, based on 4 squall line processes occured over North China during 6-7, 1986,the synoptic situations of occurrence of squall line are analysed, and have been compared with commonly convective activity. It is found that the occurrence of squall line is related to the cold air breakout in the middle levels.Before its occurrence,. the unstable energy are characteristic of larger, concentrative distribution and increased with time. Some significant results could be useful for analysing and forecasting the occurrence of squall line.