  We report the magnetoresistance, magnetothermopower and quantum oscillation
study of Sb$_2$Te$_2$Se single crystal. The in-plane transverse
magnetoresistance exhibits a crossover at a critical field $B^*$ from
semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear
magnetoresistance which persists up to the room temperature. The
low-temperature Seebeck coefficient is negative in zero field contrary to the
positive Hall resistivity, indicating the multiband effect. The magnetic field
induced the sign reversion of the Seebeck coefficient between 2 K and 150 K, .
The quantum oscillation of crystals reveals the quasi-two-dimensional
(quasi-2D) Fermi surface. These effects are possibly attributed to the large
Fermi surface which touches Brillouin zone boundary to becomes quasi-2D and the
variation in the chemical potential induced by the magnetic field.
