In this work, a pulsating extensional stress induced melt mixer was developed. The compositions, structure, working principle, and melt mixing process of the device were introduced in detail. The mixer generated a full-process pulsating extension deformation effect on materials by the eccentric vane units and arc-shaped flow channels, which greatly promoted the dispersion and mixing of the materials. Polypropylene/carbon fibers (PP/CFs) composites were prepared by the new mixer, and the effects of different rotational speeds and mixing time on the properties of the composites were studied. The results showed that proper rotational speed and mixing time effectively promoted the dispersion of CFs in PP matrix, while the fibers still had long remaining lengths. As a result, the overall properties, especially the mechanical properties of the PP/CFs composites improved. When the CFs content was 30 wt.%, at the process parameters of 20 r/min and 2 min, compared with pure PP, the tensile strength (58.46 MPa) and modulus (1962 MPa) of the PP/CFs composite were increased by 78.6% and 263.3%, respectively.