The influence of hydraulic retention times (HRTs) reduction from 25 days to 15 days on the enhancement effects of two pretreatments (thermal pretreatment and alkaline-thermal pretreatment) on the continuous anaerobic digestion (AD) of sewage sludge was studied in a long-term experiment (196 days). The operation of the semi-continuous AD fed with raw sludge or pretreated sludge was stable at the three HRTs. The methane production increased from 70.6 to 165.8 ml/L.d to 75.2-172.6 ml/L.d and the methane yield decreased from 98.9 to 234.9 mug added volatile solid to 65.6-144.9 ml/g added volatile solid when the HRT reduced from 25 days to 15 days. The two pretreatments reduced the HRT of raw sludge AD by over 40%, and the effects of the alkaline-thermal pretreatment were greater than those of the thermal pretreatment. The reduction of HRT from 25 days to 15 days increased the enhancement effects of the two pretreatments on the removal of organic matter (4.7-15.9% for volatile solid), average hydrolysis ratio (36.9-116.4%), and specific hydrolysis rate (44.1-155.6%) but decreased the enhancement effects of the pretreatments on the methane production (0.9-4.6%) and yield (4.0-15.8%), average reaction ratios (0.4-8.2%), and specific rates of the last three AD steps (0.1-13.9%). The influence of HRT reduction on the enhancement effects of the alkaline-thermal pretreatment for sludge AD was slightly greater than on the enhancement effects of the thermal pretreatment. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.