Achieving hydrothermal hydrogenation and deoxygenation of bio-oil with high water content such as microalgae-based bio-oil still faces significant challenges. Traditional upgrading processes require a large amount of H2. The objective of current research uses methanol and formic acid as a hydrogen donor to achieve the hydrothermal hydrodeoxygenation of palmitic acid (a model of microalgae-based bio-oil) to prepare pentadecane over Pt/C catalyst without the need for additional H2 supply. The research findings have shown that the H2 supply mechanism of methanol and formic acid in synergy with palmitic acid hydrothermal hydrodeoxygenation is obviously different. Formic acid is directly decomposed to produce H2, while methanol undergoes aqueous steam reforming to produce H2 in synergy with palmitic acid hydrodeoxygenation. More importantly, methyl palmitate, a typical model compound of the first-generation biodiesel, achieved efficient deoxygenation without the addition of H2 and hydrogen donors. At 300 °C, the yield of pentadecane reached 91.25 % within 150 min. The hydrothermal stability test shows that the decrease in Pt content and specific surface area (SBET), as well as the destruction of pore structure, are the main reasons for the decrease in activity of the Pt/C catalyst after cycling. The H2 production efficiency of hydrogen donors possesses a significant impact on the hydrodeoxygenation of palmitic acid and therefore more efficient catalysts need to be developed. Display omitted •Palmitic acid deoxygenation was achieved without the need for additional H2 supplies.•The rate of methanol transfer hydrogenation of palmitic acid is much higher than that of external H2.•Efficient deoxygenation of methyl palmitate over Pt/C without the need for hydrogen donors and external H2.•The yield of pentadecane reached 91.25 % in hydrothermal deoxygenation of methyl palmitate at 300 °C.