Phenolphthalein-diamine benzoxazine resin (PTBZO) was synthesized using phenolphthalein, diamine and formaldehyde and the corresponding polymer of poly(phenolphthalein-diamine benzoxazine) (PPTBZO) by thermal polymerization was obtained. The chemical structure of PTBZO and PPTBZO was characterized by FT-IR and 1H-NMR. The thermal degradation behavior of PPTBZO was studied with TG-DTG technology and the thermal degradation mechanism was also proposed by the model fitted method and non-model fitted method. At the same time, the dielectric property was also studied. The results show that the fracture of Mannich bond was the major degradation process under the low conversion rate and the introduction of phenolphthalein groups greatly improved the thermal stability of the resin. The thermal degradation reaction mechanism followed the random nucleation and subsequent growth. The complex dielectric constant was 2.65 and 0.05 corresponding to real and imaginary parts, and the dielectric loss tangent ranged from 0.02 to 0.14.