Abstract: The Longquanshan fault zone as the foreland uplift of the Longmenshan nappe zone lies at the eastern boundary of the Chengdu plain, and has long been studied by the geologists concerning the activity of the faults in this zone. The measurement of the soil gas radon may be applied to determine the location and activity of the hidden faults. The measurements of radon in this paper show that there occurs stronger fault activity on the eastern slope than on the western slope, and in the major faults than in the surrounding hidden faults in the northern part of the Longquanshan fault zone. There is a positive correlation between the radon anomaly and fault activity. The average radon concentrations within the Longquanshan fault zone are generally 9.6 times higher than the background values in this area. On the whole, the Longquanshan fault zone is a slightly active to moderately active fault zone.