Abstract: Abstract: Due to the dense materials - hemicellulose, cellulose and lignin which were difficult to degrade in the crop straws, the anaerobic decomposition couldn't decompose and utilize the crop straw quickly and efficiently. Appropriate pretreatments were needed to promote the efficiency of crop straw digestion. Lots of studies at home and abroad had shown that alkali pretreatment of crop straw had many advantages, such as having short processing time, good gas production and so on. To improve the gas production ability meanwhile reduce costs and security risks, relatively low cost and high security CaO was chosen as the pretreating agent in this study. Corn straws were pretreated by three different weight percentages (3%, 5% and 7%) of CaO retting for three different time (three days, five days and seven days) in an incubator chamber at (25±1)℃, and the effects with different conditions of CaO pretreatment on anaerobic digestion of corn straws for variation of components and anaerobic digestion performance (under temperature of (38±2)℃) were studied. A control set with untreated corn straws was also done to compare the difference with the treated ones. The experiments were set up in triplicate. The results showed that having been pretreated by CaO, the total content of hemicellulose, cellulose and lignin in corn straws was decreased, and the daily gas production and cumulative gas production got a substantial increase compared with the control. In addition, the starting time and digestion time was shortened significantly. The cellulose degradation effect of 5% and 3% of CaO with 3 days and 5% of CaO with 7 days were better in these nine pretreatments, and the degradation rates were 60.67%, 44.12% and 39.28%, respectively. The cumulative biogas production of 5% and 3% of CaO with 3 days were higher than others, and their unit dry mass gas production were 381.87mL/g and 369.00mL/g, which were 136.85% and 128.91% higher than the control. The digestion time of 80%of maximum gas production of all the treated groups was shortened by 3 to 10 days compared with the control. Most of the treated groups had significantly improved the TS and VS removal efficiency. The TS removal rates of 5% and 3% of CaO with 3 days and 3% of CaO with 5 days were 66.45%, 64.58% and 61.37%, respectively. The VS removal rates of 5% of CaO with 5 days and 3% and 5% of CaO with 3 days were improved by 10.20%, 7.95% and 7.18%, respectively. Taking daily biogas production, cumulative biogas production, anaerobic digestion time and the TS and VS removal rate into account, the weight percentage 3% of CaO with 3 days pretreatment was the best, compared with the control. The cumulative biogas production of this best treatment increased by 136.85%, and the fermentation time was shortened by 5 days. Meanwhile, the removal rates of TS increased by 21.72% and the removal rates of VS increased by 7.18%. Therefore, the weight percentage 3% of CaO with 3 days pretreatment exhibited the best effect in all of the cases, and it improved the biogas production rate and the biogas production efficiency of anaerobic digestion of corn straws effectively.