Abstract: In this paper, the hat-shaped structure preforms of carbon fiber unidirectional prepreg laminate were pre-formed with the hot stamp forming device. The effects of temperature and forming speed were investigated by measuring the apparent mass, thickness and fiber deflection angle of preforms. A relatively great friction force between prepreg layers is observed at low temperatures due to the low viscosity of the resin, which causes the wrinkle on the surface of hat-shaped structure preforms and angle deflection of fiber by the inter-ply shearing. At high temperature, the fluidity of the resin is enhanced under the pressure. Not only the thickness of preforms are reduced, but also the deflection angle of fiber is increased due to the ability of resin to bind the fiber being diminished. As the increase of forming speed, the deflection angle of the fiber shows an increasing trend. Therefore, the temperature and forming speed must be controlled to improve the preforming quality.