Abstract: More than 100 wells have been drilled in the Yitong graben since 1987. Heowever, there is a considerable difference between the results respectively from previous calculation and exploration. By using the method of the authors, the generation expulsion and accumulation of hydrocarbons were computed in the area. The result showed that the present discovered resources is only 10% of the total accumulation. The main reasons were explained as follows: sediments are badly sorted because of being near sourceland, which resulted in a little difference in properties between reserviors and mudstones, and lack of regional seals or direct caprocks. Due to large space of secondary migration most of hydrocarbors was absorpted or escaped in the horizons and was difficult to form pools.