Biochar has unique physicochemical properties of being rich in carbon, being alkaline, and exhibiting a highly porous structure, which can adjust features of different systems. A 90-day microcosm incubation experiment was performed to investigate the effects of corn straw biochar on the process, properties, nutrient contents, and CO2 emissions during corn straw composting. There were four treatments, including control (CK), 5% biochar addition (B1, as mass fractions of biochar), 10% biochar addition (B2), and 20% biochar addition (B3). The results showed that biochar significantly increased the temperature rise rate and temperature peak of the straw maturation system, and promoted straw decomposition. Biochar increased the pH of the microbial active period, and the electrical conductivity (EC) value of the straw decomposition system, which provided a more suitable environment for microbial degradation of the organics. Further more, biochar decreased the organic matter content, increased the total nutrient content of the straw decomposition system, and improved the quality of the straw decomposition products. In addition, nitrogen (N) content was not changed by increasing amount of biochar; however, both phosphorus (P2O5) and potassium (K2O) content were significantly increased. Compared to control, the content of P2O5 and K2O in B3 treatment was increased by 0.2% and 0.9%, respectively. Biochar addition could improve CO2 emission of the straw decomposition system. The CO2 emission was consistent with the trend of temperature change, which provided solid evidence that biochar improve the degradation of organic matter by microbes in the system.