Abstract: At present, most biogas projects in China use crop straw and animal manures as the main raw materials, however, there is still a lack of reference and support for the process control of the actual biogas project, especially for the control of the feed mix of biogas plants. In this study, two kinds of common materials, corn stalk and cow dung, were used as the influent with different mixed ratio under the medium temperature conditions. The mixed ratio of straw and cow manure (S:CM, as TS ratio) was adjusted to 1:1, 1:3, 3:1, while the reactors with straw and cow manure were as the controls. The CSTR started at the condition of TS=3%, then the TS was increased to 6% and 8% sequentially. The effects of different feeds on the operation characteristics of biogas engineering such as gas production, methane content, pH value, ORP (oxidation reduction potential), volatile acids were systematically discussed. The results showed that the biogas production of the reactors with straw or cow manure was significantly lower than that with mixture. And the biogas production rate increased with the increasing of TS. Under the conditions of TS=8%, the reactor with influent of S:CM=3:1 got the peak biogas production rate of 0.78 L/(L·d) after 130 d running, and that with influent of S:CM=1:1 got its peak biogas production of 0.76 L/(L·d) after 150 d running. As the TS in the reactor was increased to 8%, the pH values in the five reactors did not change significantly after increasing the concentration, indicating that there was still a certain buffering capacity in the reactor. Then, with the continuous operation of the reactor, the pH value in the reactor gradually decreased on the 170th day, and the pH value in the five reactors decreased to about 6.5 in 180 days. The pH value decrease led to a decrease in the gas production rate of the reactors. The biogas production rate decreased to 0.6 L/(L·d) with a decreased pH value to 6.5. This was mainly because under the constant stirring power condition, as the concentration of the materials in the reactor increased, the stirring speed was reduced, and problems such as stirring dead zone and scum were occurred in the reactors, resulting in acidification in the reactor. This in turn caused the system to be unstable overall. During the startup, the ORP of all the five reactors showed a slowly rising trend. After 172 d running, the ORP in the reactor with S:CM=1:1 rapidly increased to above -300 mV. Overall, the concentration of VFAs in the anaerobic system increased with the increasing proportion of straw in the feed-in materials. Moreover, the accumulation of propionic acid occurred and became more serious as the proportion of straw in the feed-in material increased. This result showed that the biogas plants with more straw might ran at more unstable conditions, compared with those feeding in mixture of straw and manure.