Abstract: Abstract: In order to improve the volumetric gas production efficiency of pig manure in continuous biogas project and reduce the heat loss during its feeding and unloading process, the high concentration and small volume feeding method is adopted in this study. The total solid content (TS) of swine manure collected from dry cleaning mode is usually greater than 20%, and thus the manure can be used in not only wet but also the semi-dry or dry anaerobic digestion (AD). Previous reports showed that the dry AD mode can obtain better biogas production than wet AD process with a TS lower than 10%, and can also reduce the volume of dilution water and biogas slurry. Recently, Chinese government is pushing ahead the dry cleaning mode in the livestock and poultry farms, and thus there will be more and more "dry manure" with TS of above 20% produced from these farms. Therefore, it will be more necessary to study the semi-dry or dry AD process of swine manure. In present assay, a semi-continuous AD experiment up to 100 days was carried out to investigate the interaction between feedstock TS and the hydraulic retention time (HRT). There are 3 sets of reactors, and the feedstock TS is set as 10%, 12% and 14%, respectively. All the reactors start up from HRT of 25 d, and then the HRT is decreased to 20 and 18.5 d in step by step. The descent process of HRT will cause an increase of organic loading rate (OLR), and thus the reactor state and the biogas production will be influenced. The real-time biogas yield, volumetric biogas production rate (VBPR) and the buffer capacity of each reactor were investigated. The experimental results showed that all the 3 sets of reactors obtained a maximum biogas yield in the continuous AD test when they operated in the stage of HRT of 25 d, and there was no significant difference on the biogas yields among the 3 sets of reactors, around 460 mL/g which reached 86% of the maximum biogas yield of swine manure obtained from the batch AD process also carried out in present assay. The VBPR of each reactor reached the maximum value at the HRT of 20 d, and the highest VBPR of 2.29 L/(L·d) was observed at the TS of 14% and the HRT of 20 d. When the HRT decreased to 18.5 d, the biogas yield and the VBPR showed a larger reduction, mainly due to that the higher OLR had exceeded the bear capacity of these reactors. Moreover, the VFA/TA values were close to or exceeded 0.4 at HRT of 18.5 d, which suggested these reactors had a weaker buffer capacity at this time and might be running to the acidification. The optimum operating combination of feedstock TS and HRT could be selected by synthesizing the 3 factors of biogas yield, VBPR and the buffer capacity, and the feedstock TS of 14% and HRT of 25 d were recommended according to the present experimental results. The present study can provide a help to the practical biogas engineering for the matching of feedstock TS and HRT.