The dynamic rheology and thermomechanical properties of poly(trimethylene terephthalate) (PTT)/short carbon fibre (CF) composites at different mechanical states were investigated by a rotational rheometer and a dynamic mechanical analyzer (DMA). At molten state, the composite melts were pseudo-plastic fluids, and the complex viscosity of the composite melts decreased much with increasing CF content because of the poor adhesion at the fiber/matrix interface. The viscous behavior was predominant rather than elastic behavior in the composites melt and viscous behavior was increased with increasing CF at low shearing frequency. An apparent slope change in storage modulus and loss modulus plot suggested that a structure change occurred in the melt that was dependent on shearing frequency. At glassy state, the storage modulus increased with increasing CF content, suggesting that CFs had good reinforcing effect on PTT. At glass transition region, the increasing loss modulus indicated a better toughness of the composites, and the elastic behavior was predominant rather than viscous behavior. Moreover, the glass-transition temperatures of the composites increased with 10% CF content. The composites have larger cold-crystallization rate than pure PTT.