In this paper we present a timing detector which has low detection threshold and is radiation-resistant. The photons induced by radioactive beam ions passing through a thin plastic-scintillator foil BC422, emit from the foil center corresponding to one focal point of an aluminum ellipsoidal mirror and are reflected to another focus point at which the cathode of a photomultiplier tube locates. A time resolution of about 115 ps can be achieved for 12N of 58.5 MeV/u and 130 ps for 17B of 43.7 MeV/u. The counting rate up to 108 packets per second (pps) is allowed.