Nonlinear seepage movement characteristics of fluid in underground coal gasification (UCG) are studied in this paper. On the basis of a model experiment, via the analysis of seepage field distribution and its change patterns in the process of coal gasification in the gasifier, the mathematical models of three-dimensional unstable and nonlinear seepage are established. The selection method of choosing major model parameters is illustrated. Solution to the mathematical models is made through the finite element method. This paper also analyzes and discusses the simulated calculation results. The gas pressure declines along the slope of the coal bed gradually, whose range of drop varies greatly. In the combustion cavity zone, the pressure changes slightly; in the burning zone, the fluid pressure drops sharply; and when it comes to the loosening zone, the value of the pressure stands at the minimum. The simulated results indicate that the relative differentiations among all the measuring points in the gasifier range from 10% to 18%. The calculated results agree virtually with measured ones, which supports that the numerical simulation of seepage field in the gasifier is reliable. (C) 2003 Elsevier Science B.V. All rights reserved.