本文从森林地表火蔓延的本质入手，提出了火场扩展的“点义源原理”，并在此基础上建立了一套完整的确定动态火场蔓延边界的新模型。对若干算例试算的结果表明，模型对可燃物类型、地形和气象条件复杂变化下的火场边界的确定，能获得相当合理的图形显示的结果。 Based on the essence of the spread of forest surface fire, this paper puts forward “the principle of point source” which is extended by the fire field and establishes a new set of complete model to determine the boundary of dynamic fire spread. The experimental results of some numerical examples show that the model can obtain quite reasonable results of graph display on the determination of the boundary of fire field under the complicated changes of combustibles types, topography and meteorological conditions.