Abstract: Abstract: China is a large agricultural country, and crop straw is one of the main components of the agricultural waste. Anaerobic digestion (AD) to produce biogas is an efficient way for straw utilization. Utilizing straws as raw materials in biogas projects are usually need to be stored for a period of time after harvesting. Different storage time has effects on the biochemical methane potential (BMP) of straws. In this paper, we analyzed the change of physical & chemical properties and effects on the BMP of maize (Zea mays) straw under the circumstances of storage time for 0, 2, 5, 10, 20, 30 and 45 days piled naturally with room temperature. Physicochemical properties, such as sensory properties, total solid (TS), volatile solids (VS), pH, lignocellulose, chlorophyll, volatile fat acid (VFA), ammonia nitrogen, etc., were analyzed under different storage time. The results indicate that the properties of straws tended to be stable after 20 days of storage. As the storage time increased, the water content decreased, and the straw looked like hay after 20 days of storage. The soluble characteristics extracts, such as soluble chemical oxygen demand (SCOD), reducing sugar and VFA, decreased rapidly during storage for 0 - 5 days, and then were stable after around the 5th days. The content of lignocellulose in the storage process was generally increasing. The BMP tests on straws with different storage time showed that the BMP value with different storage time for 0, 2, 5, 10, 20, 30 and 45 days were 325.49, 315.35, 297.85, 296.71, 295.25, 287.83 and 291.01 mL/g (in VS), respectively. As the storage time increased, the BMP showed an overall downward trend, and the methane production rate of storage time for 0 - 5 d was decreased significantly, while the rate tend was stable after storage for 10 days. The BMP with storage for more than 5 days (10 d, 20 d, 30 d and 45 d) were roughly similar. During the whole AD period, the cumulative methane production trends of the respective experimental group were similar, while the daily methane production showed a higher peak (AD period at 1.25 - 6 d) and a lower peak (AD period at 6 - 10 d). After the two peaks, the daily methane production tended to be stable. The biodegradability of each group showed that the fresh straw (without storage) had the highest biodegradability, and the biodegradability decreased with the storage time lasting. With the reducing of moisture content and sugar content during the storage, the metabolism of plant cells and microorganisms were weakened or even die. Thereby, extending the storage time had slight effect on each property and BMP of straw. There was a significant TS loss during the storage procedure because of the plant cell respiration and the microbial biochemistry. Considering the TS loss, the methane potential loss reached to 36.48%. We concluded that maize straw storage for different lengths of time had a significant effect on the BMP, mainly due to the influence of various chemical components, especially moisture lignocellulose, SCOD, reducing sugar and volatile fat acid. The results can provide reliable references for the straw biogas projects operation.