The anaerobic co-digestion of pig manure (PM) with dried maize straw (DMS) was studied at 35 °C with a volatile solid (VS) ratio (VSPM/VSDMS) of 1:2 in a continuously stirred tank reactor, and the digestions of mono-PM and mono-DMS were evaluated under the same conditions. The organic loading rates (OLRs) of 2, 3, and 4 g VS/L/d were studied and found to correspond to hydraulic retention times (HRTs) of 60, 40, and 30 d, respectively. Under the condition of long HRT and low OLR, PM could be degraded completely. The co-digestion of PM with DMS showed the most stable performance in TAN, whereas TAN in mono-PM increased with the increase of OLR. The specific methane yield (SMY) did not have a linear correlation with OLR, since HRT changed with different OLR. The maximum average SMY in the co-digestion reactor was 272 mL/g VS-fed at an OLR of 3 g VS/L/d and an HRT of 40 d. The SMY in mono-DMS digestion was the lowest and it decreased with the increase of OLR.