In this study, the effects of inorganic (HCl, HNO3 and H2SO4) and organic (HCOOH, CH3COOH and HOOCC-OOH) acid pretreatments on the hydrothermal liquefaction (HTL) of municipal secondary sludge (MSS) were investigated. The results showed that all acid pretreatments could increase bio-oil yield by changing the proportion of organic matters and ash content in feedstock and thus facilitating the conversion of water soluble product to bio-oil. A maximum bio-oil yield of 26.75 wt% was obtained from the HCl pretreatment which increased 7.5 wt% compared to the blank experiment. Moreover, the inorganic acid pretreatments showed a better performance in terms of enhancing bio-oil quality as their higher heating values (HHVs, 31.83-36.04 MJ/kg) were much higher than that from organic acid pretreatments (24.57-29.22 MJ/kg). The characterization analysis of bio-oil indicated that the HCl pretreatment was most beneficial for the formation of light fraction and increment of hydrocarbon.