A process for separating and recovering naphthenic acid from fractional oil of petroleum includes preparing the deacidifying solvent with 5-45% of aalcohol concentration and 1-8 wt.% of sodium hydroxide concentration from the mixture of one or two or more of those chosen from C1-C3 alphatic alcohols, sodium hydroxide and water, mixing it with said fractional oil at 20-90 deg.c, laying asie, separating oil phase from water phase, and recovering naphthenic acid. Its advantages are high deacidifying rate up to over 95 wt.%, high purity of naphthenic acid up to over 90 wt.%, and no colour change of deacidified oil on the whole.