Abstract: The Emei Mountain basalt of a hydropower project in the southwest of China is special in its weathering characteristics. There exists abnormal weathered belt inner rock mass (called differential weathering). Differential weathering is much different from normally weathering rock mass in its apparent, distribution, chemical characteristics and environment involved. The distribution characteristics of differential weathering rock mass are analyzed. And the formation mechanism is studied by analyzing chemical characteristics, valley evolution and near surface action theory. The main conclusions are drawn that the differential weathering does not weaken with the increase of the horizontal depth. The distribution is controlled by long tension fracture, and highly weathered near fracture. It is much redder within 5~15 cm of the tension fracture. Chemical weathering occurs in differential weathered rock including decrease of Fe²⁺ and increase of Fe³⁺. But there is no obvious decrease of Na, K, Ca, Mg, Al and Si. The controlling discontinuity appears near surface action characteristics, and differential weathering rock mass is formed by near surface action.