In order to improve the storage stability of bio-oil from pine wood pyrolysis with fluidized bed reactor, methanol in different proportions (3 wt%, 6 wt%, 9 wt%, 12 wt% and 15 wt%) was added into bio-oil. The changes of physicochemical properties of bio-oil samples such as water content, pH value and viscosity and the aging chemical reaction mechanism during 35 days storage were investigated. Water content and viscosity of the methanol addition group were obviously lower than that of the raw bio-oil, which indicates that methanol could effectively inhibit the generation of water and greatly reduce viscosity during storage. When the methanol content was higher than 6.0 wt%, pH value began to increase with the storage duration. GC-MS and FT-IR analysis results showed that the component and structure of bio-oil were changed. During the storage, some reactions, e.g. polymerization reaction and aging reactions, occurred in bio-oil samples and the addition of methanol could delay the aging process of unimolecular elimination reaction (E1 elimination reaction). Phenols of methanol/bio-oil could react following three possible reaction mechanisms according to guaiacols ortho-methoxyl substitution, unimolecular nucleophilic substitution (S(N)1) reaction and bimolecular nucleophilic substitution (S(N)2) reaction of ortho-hydroxyl substitution of catechol. As a conclusion, the addition of methanol could improve the bio-oil storage stability and be beneficial for its application.