Carbon-based CsPbBr3 perovskite solar cell is an emerging inorganic perovskite solar cell with the advantages of simple fabrication process and excellent stability. However, power conversion efficiency of carbon-based CsPbBr3 perovskite solar cells is still unsatisfactory up to now, as the direct contact of the CsPbBr3 with carbon is plagued with interfacial recombination sites and undesirable hole extraction barrier. Here, we report an effective strategy that employs poly (3-hexylthiophene) (P3HT) to modify the CsPbBr3/carbon interface in carbon-based CsPbBr3 perovskite solar cells and enable higher efficiency. The systematic tests and analyses demonstrate that the P3HT interlayer can remarkably suppress the charge recombination and enhance the hole extraction capability via formation of favorable energy level alignment between CsPbBr3 film and carbon electrode, and passivation of the surface defect states of CsPbBr3 film. As a result, the carbon-based CsPbBr3 perovskite solar cell with P3HT interlayer achieves a high conversion efficiency of 6.49%, exhibiting an increase by 27% compared to pristine device. Moreover, the carbon-based CsPbBr3 perovskite solar cells with P3HT interlayer exhibits excellent stability in ambient air with almost no change in the power conversion efficiency of the unsealed device over 40 days.