By the technique of in vitro gas production, this paper studied the in vitro fermentation characteristics of different kinds of crop straws and their combined utilization. The results showed that the theoretical maximum gas production and the gas production rate of silage corn were increased by 19.1% and 8.2%, respectively, compared to corn stalk. These two indices of mixed crop straws were higher than those of single crop straw, and significant positive interactions in gas production were observed after 48 h of incubation. It was concluded that a combined utilization of silage corn with different kinds of crop straws could be an efficient way to improve the nutritional characteristics. The optimum ratio of the mixture was 25/75 or 50/50 when silage corn mixed with wheat or rice straw.