As a liquid product from the fast pyrolysis of renewable biomass, bio-oil has a great potential use as a fuel or high-value chemicals resource, but it is accompanied by inherent drawbacks, such as strong corrosiveness and chemical instability. A facile method for bio-oil upgrading by zero valent metals (Al, Fe, Mg, and Zn) at ambient temperature and pressure was reported for the first time in this work. The chemical features of the raw and upgraded bio-oils were analyzed by Nuclear Magnetic Resonance (NMR) and GC-MS. The results show that the zero valent Zn favorably demonstrated improvement of the quality of bio-oil. In the upgraded bio-oil, the CO compounds are reduced from 9.8 to 3.1 mol%, and the pH value was elevated from 3.53 to 4.85, which significantly increased the chemical stability and decreased the corrosiveness of bio-oil. The possible formation mechanism of 13 newly formed compounds in the upgraded bio-oil was explored, and most of them were found to be derived from hydrogenation of CO compounds. This work provides an economical and environmentally friendly approach for bio-oil upgrading.