A series of poly(butylene terephthalate) (PBT) based composites with modified potassium titanate whiskers (MPTW) were prepared through in-situ polymerization. The surface properties and loading amount of MPTW were studied about their effects on the polymerization characteristics, microstructure, thermal stability, crystallization behavior, mechanical properties, friction-reduction, and wear-resistance of the PBT-based composites. The results show that the MPTW is in-situ chemical grafted with PBT, resulting in good dispersion and blurry phase interface. The in-situ grafted MPTW shows little effect on the thermal stability of PBT, but affects its semi-crystalline time. When the MPTW loading amount is 4%, mechanical properties of the composite reaches optimum, with a doubling of tensile strength and a tripling of impact strength compared to pure PBT. Additionally, the friction-reduction and wear-resistance of PBT are significantly improved with the addition of MPTW. Overall, the high-strength and wear-resistant PBT composites show potential application in the field of structural engineering plastic and mechanical transmission parts.