The phase transition of paraffin powder mixed in wellbore concrete is generated by baking it, so that the paraffin powder is melted. The influence of baking temperature and paraffin dosage on phase transition degree of paraffin powder and the carbonation resistance of wellbore concrete was studied. Combined with microscopic testing methods such as scanning electron microscopy, the microscopic morphology of phase-transited paraffin films within the cement matrix and the mechanism by which they enhance the carbonation resistance of cement-based materials were analyzed. The results indicate that when baking temperature reaches 80 °C, the paraffin particles in the cement-based material can melt into viscous droplets. Partially melted paraffin particles condense within the internal pores of the cement matrix upon cooling, sealing the transmission pathways of CO₂ gas into the cement-based material. Fully melted paraffin droplets flow within the internal pores of the cement matrix and, upon condensation, firmly adhere to the pore walls, forming a relatively dense and smooth film. This film isolates CO₂ gas contact with the cement matrix, significantly improving carbonation resistance of the cement-based material. When the paraffin dosage exceeds 3% and the baking temperature surpasses 100 °C, the phase transition of paraffin powder achieves a fully carbonation-resistant effect.