The influence of the length and concentration of carbon (CF) and glass (GF) fibers on the rheological, physicomechanical and thermal properties of high-temperature polyphenylene sulfone thermoplastic with a filler content up to 40% (by mass) was investigated. Carbon and glass fibers 0.2- and 3-mm long were used as fillers. The introduction of CF and GF leads to a noticeable decrease in the melt flow index and the impact strength of polyphenylene sulfone, while the yield stress disappears on the stress-strain diagram, which is caused by a decrease in the material’s ability to plastic deformations. With an increase in the fiber content, the elastic modulus and tensile strength of the composite increase both in bending and in tension. A significant increase in heat resistance occurs with an increase in the CF content, and samples with a CF content of 0.2 mm have a higher mass loss temperature. In the case of composites with GF, there are no significant changes in heat resistance.