Using a fast decomposition agent produced by Hunan Taigu Company and weight lose method adopted, effect of applying dosage on decomposition of maize straw was studied at normal atmospheric temperature. There were 4 treatments tested:（1） water CK,（2） 4g fast decomposition agent,（3） 6g fast decomposition agent and（4） 8g fast decomposition agent. The results showed that the decomposition rate of maize straw was increased with the increase of dosage of fast decomposition agent. After 180d, the best effect was gotten in the 8g fast decomposition agent treatment, and the decomposition rate of maize straw was 38.1%, which was24.9% higher than the CK. The decomposition speed was the highest and the decomposition effect was the best when trial time reached 20days.