Fuel moisture content is a good index to assess fire potential. Fire occurrence and spread are limited by meteorological conditions on large degree, and global warming have influences on forest fires. By using daily meteorological data and Canadian Fire Weather Index (FWI) System, three fuel moisture codes had been calculated during 1972-2005 for Daxing′an mountains in Inner Mongolia, such as Fine Fuel Moisture Code (FFMC), Duff Moisture Code (DMC), Drought Code (DC). Long-time trends of moisture codes and its effects on fire occurrence and spread were analyzed respectively in spring fire-prevention period, autumn fire-prevention period and summer non-fire-prevention period. The results show that the seasonal averages of FFMC, DMC and DC have an increasing tendency during 1972-2005, and FFMC, DMC increased significantly in summer. That means the fuel-dryness conditions during 1972-2005 had an increase-tendency and especially expressed in surface-layer and upper-duff layer fuel in summer. The fuel-dryness condition changes may be a reason for frequent lightning fires especially in summer in recent years. In addition, the duff fuel-dryness severity increased fire control and mop-up difficulties. In future, the fuel-dryness conditions in Daxing′anling forest region will become more severe under a warmer climate.