This study examines how to stop the pyrolysis of fir needles, birch leaves, aspen twigs and their mixture using the minimum volumes of water. The combustion of forest fuels is suppressed by spraying water on their surface. The temperature of thermal decomposition is monitored throughout the layer of forest fuel by thermocouples. A high-speed camera and optical techniques allow us to study water spraying and its interaction with forest fuels. Finally, the study specifies the ranges of the minimum water volumes and the times of ending of the thermal decomposition of forest fuels. When analyzing the energy balance in the thermally decomposing forest fuel, a mathematical expression is formulated to predict the water volume sufficient to suppress thermal decomposition of forest fuel. This expression takes into account the ratio between the heat energy spent on water evaporation in pores of forest fuel and the heat energy of the reacting layer of forest fuel. The obtained dimensionless factor considers the main parameters of water spraying and the properties of forest fuel. This factor enables us to apply the research findings to forest fuel in various regions of the world.