Abstract: Abstract: Grassland is an important land resource in China. Grassland fire is one of the worst natural disasters which threaten the safety of herdsmen's lives and properties and ecological environment in the country. Due to vast territory, rich combustible and low rainfall of grassland, grassland fire has the features of suddenly happening, huge destruction and large influence area. As to organize fire suppression efficiently and reduce losses from fires, it has important significance to conduct the research on spreading laws of grassland fire under different influencing factors and on forecasting spreading trend of grassland fire duly.　　The researches on behaviors of grassland and forest fires at home and aboard are mainly based on field observation and laboratory experiments, several important factors being seized through those. Moreover according to laws of statistics and physics, corresponding mathematical models are developed lastly for estimating the properties of wildfires. Researches on spreading laws and models of grassland and forest fires are mainly focused on forest fires at present. Grassland fires are often analyzed on the basis of the existing models of forest fires. In more recent years, it has been adopted by researchers gradually to forecast behaviors of grassland and forest fires with computer simulation.　　FDS (Fire Dynamics Simulator) is the software of computational fluid dynamics to simulate fluid-flow in fires developed by NIST (National Institute of Standards and Technology) in USA. Numerical approach has been developed by the software to solve Navier-Stokes equations of flow of low Mach number. Afterwards gas and process of heat transfer in fires have been calculated especially, through those the producing and move of gas in fires could be tracked and forecasted, so as to calculate the producing and spreading of fires.　　In the paper, the simplified model of grassland fire has been created for a practical pasture with FDS firstly. Correlation analysis has been conducted on the parameters that influence the calculation accuracy such as the setting of mesh density, height of computed field and size of computed field. Secondly, based on the results of correlation analysis, numerical calculation has been conducted on the spreading laws of grassland fire in the condition of different wind speed. Calculation results and experiment results of field have been compared, which proves the correctness of the fire model. Finally, three-dimensional numerical research has been conducted on the spreading laws of grassland fires in the conditions of different ambient temperature, different gradient and different water content of combustibles of the model; the influences of different factors on the spread of grassland fire have been analyzed and compared; laws of heat release rate, combustion rate of fires in different factors have been obtained. As a result, it provides the theoretical basis on formulation of effective measures to fire prevention and extinction timely.