A compound additive consisting of methanol, N,N-dimethylformamide (DMF), and acetone was obtained with the A-Optimal mixture design of Design-Expert. How the solvent effected the stability of bio-oil was analyzed on the basis of viscosity, moisture content, and pH. Bio-oil with the optimal compound additive (1 wt %,methanol, 5.064 wt % DMF, and 1.940 wt % acetone) had a low viscosity and moisture content and high pH after aging (80 degrees C for 24 h). The corresponding property values were 4.36 mm(2)/s, 24.03%, and 4.49, respectively, and the result was better than bio-oil with a single solvent. Gas chromatography mass spectrometry (GC-MS) analysis revealed that the phenol contents in all of the compounds in bio-oil were high. After aging, the contents, of sugars and esters increased and several chemical compounds, such as, 2-ethoxy-5-(1-propen-1-yl) and 5-hydroxy-2-methylbenzaldehyde, disappeared in bio-oil with the compound additive. Elemental analysis showed that the contents of O, and N increased after the addition of the optimal compound additive. The compound additive exerted a positive effect on the bio-oil during storage.