In the context of the difficulty of high-value recycling and treatment of thermoset composite waste, thermoplastic composites have a tendency to replace thermosetting composites, and exploring the thermal degradation mechanism of thermoplastic composites is conducive to guiding the use of thermoplastic composites under different working conditions and the subsequent recycling and degradation. The thermal degradation behavior of polyacrylic acid thermoplastic resin (RE) and thermoplastic resin glass fiber composites (RE-FRP) under different heating rates was compared and studied by a thermogravimetric analyzer under nitrogen atmosphere. The results show that the temperature of RE-FRP composites is higher than that of RE pure thermoplastic resin when the thermal degradation rate reaches the maximum. The activation energy of thermal degradation of the RE system measured by the Kissinger method was 196.76 kJ/mol, which was 232.91 kJ/mol less than that of the RE-FRP system. The Flynn-Wall-Ozawa method was used to calculate the activation energy, the apparent activation energy of the degradation reaction of the RE pure resin system fluctuated little with the change of degradation degree, the degradation reaction mechanism was simple, while the apparent activation energy of the RE-FRP system increased with the increase of degradation degree, and the degradation reaction had a more complex mechanism. The study shows that the thermal stability of RE-FRP thermoplastic resin is improved by combining with glass fiber.