Abstract：A noticeable deep overpressure system occurs below 4,400 m depth in the well Pen-1 West subbasin in the interior of the Junggar basin. Studies of the geological, well log and geophysical data in the area show the following：the overpressure interval consists mostly of alternating beds of sandstone and mudstone；the mudstone interval is mostly a few to more than ten meters thick with a porosity of <8% and has been fully compacted; the slurry density increases greatly, being far higher than 1.2 g/cm3; both the sandstone and mudstone intervals have higher acoustic transit time and lower resistivity, and the stack seismic velocities near the wells also have abnormally lower values; the top of the overpressure interval is irregular in shape and penetrates the bed and macroscopically usually in a certain depth range. Analysis indicates that the lack of compaction and coal beds are not the cause for the lower velocity in the overpressure interval and that hydrocarbon, especially natural gas, generation makes the greatest contribution to the present overpressure generation.