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Restricted by their molecular structure defects, poly(methylphenylsiloxanes) usually exhibit a relatively low thermal stability, thus limiting their application in high-temperature areas. In this paper, we introduce a cost-effective synthesis method to prepare poly(methylphenylsiloxanes) (PPMS-M) with methyl?phenyl mixed cyclic monomers as raw materials. The molecular structure characterization shows that PPMS-M contain abundant phenyl groups, and the phenylsiloxane units are evenly distributed among methylsiloxane segments. The thermal degradation kinetics are systematically studied with the Flynn?Wall?Ozawa method. It shows that PPMS-M exhibits much higher degradation activation energy than ordinary poly(methylphenylsiloxanes) (PPMS-PD) does, which is prepared by 2,4,6-trimethyl-2,4,6-triphenylcyclotrisiloxane (P3) and octamethylcyclotetrasiloxane (D4). The thermogravimetry?Fourier transform infrared characterization shows that the degradation process of the phenyl group in PPMS-M occurs at temperatures of 100?200 °C higher than those for PPMS-PD. PPMS-M exhibits good thermal stability and a low glass transition temperature. Our method would be applied to cost-effective synthesis of other high-performance functional polysiloxanes.