A valve stem assembly for clamping tire pressure detector has a valve stem and a fastener. The valve stem has an outer section and an inner section. The inner section has a distal end, an outlet hole, a sidewall and a side fixing hole. The distal end has a domical surface. The outlet hole is centrally formed through the domical surface. The side fixing hole is formed in the inner section, is oblique to the outlet hole and communicates with the outlet hole inside the inner section. The fastener corresponds to the outlet hole and the side fixing hole and is selectively and detachably and selectively screwed into the outlet hole or the side fixing hole to hold a tire pressure detector on the valve stem assembly.