Hole conductor-free, fully printable mesoscopic perovskite solar cells with carbon electrode (PM-PSCs) have a good commercialization prospect because of low-cost production and high power conversion efficiency (PCE). Currently, the problem of improving the stability of perovskite solar cells (PSCs) is very important for commercialization. Interfacial engineering using polymers is an effective method to improve the efficiency and stability of PSCs. We prepared PM-PSCs by treatment with the toluene solution containing PS on the carbon electrode of the PM-PSCs, which would improve the charge transfer and greatly suppressed charge recombination in polystyrene (PS)-treated PM-PSCs, and also could greatly prevent degradation of perovskite into PbI2. The PS-treated PM-PSCs showed higher stability to light and moisture than the control, and the PCE was improved from 11.31% (control) to 14.03%.