Anaerobic fermentation with biogas as an energy source is influenced by the presence of heavy metals. However, the availability of the heavy metals is dependent on the digestion temperature. In this study, the impacts of Cd on the characteristics of biogas, substrate biodegradation, and enzyme activity during anaerobic co-digestion were investigated under varying digestion temperatures. The results showed that 1 mg/L initial Cd concentration improved cumulative biogas yields by 404.96%, 16.93%, and 5.56% at 55 °C, 45 °C, and 35 °C, respectively. In contrast, at low temperatures (25 °C), the yield decreased by 0.77%. In the 55 °C group, Cd addition improved the activity of cellulase (p < 0.05) and coenzyme F420 (p < 0.01). The total chemical oxygen demand (COD) during the peak period and the transformation of hydrolytic organic components into volatile fatty acids (VFAs) influenced the CH4 and biogas yields. There were no significant differences in cellulase, dehydrogenase, and coenzyme F420 activities with or without Cd addition when the digestion temperature was 45 °C, 35 °C, and 25 °C. Therefore, thermophilic digestion is recommended for the efficient degradation of Cd-contaminated biowaste. Moreover, the impact of metals on the performance of anaerobic digestion should be considered together with temperature conditions in future research and practice.