The invention discloses a constant pressure valve and a seepage experimental system. The constant pressure valve comprises a valve body which is internally provided with a valve chest. The valve chest is internally provided with a valve plug. One end of the valve plug is provided with a pressure controlling mouth and the other end of the valve plug is provided with a straight-through transfusion mouth. The side wall of the valve body is provided with a side-through transfusion mouth. The valve plug can slide between the pressure controlling mouth and the straight-through transfusion mouth. One end of the valve plug which is opposite to straight-through transfusion mouth is provided with a core hole. One side which is opposite to side-through transfusion mouth is provided with a wedge groove. The core hole is communicated with the wedge groove. The seepage experimental system can provide the required given pressures by a constant static pressure device and respectively controls the seepage pressure of an inlet and a back mouth of a coal rock sample by an entrance pressure valve and a back-pressure valve, thus achieving that the differential pressures of two end faces of the sample is constant and realizing the seepage experimental research of gas-liquid medium in coal rock in the different pressures and differential pressures.