State-of-the-art approaches to forest fire spread are based on either 2D numerical simulations of trees on GIS or rough 3D visualization. In this paper, we approximate the tree form by dynamically changing sets of tree-shape modules according to the morphological structure and wind fields. Guided by finite state machine, we define the states of equilibrium, heating, pyrolysis, cooling and destruction of tree-shape modules. Interactions between tree-shape modules drive the state transfer to achieve forest fire spread. Additionally, Loose Quadtrees are adopted to the spatial distribution of trees, which allows us to maintain the visual fidelity of the representation while rendering the forest fire spreads in real-time. Our method allows us to construct the Jiufeng forest example about 10km x 10km extent at interactive rates. The capabilities of tree-shape modules and forest fire spread visualization are demonstrated by numerous examples. Display omitted •Use FSM to define the state and transition rules of the tree-shape module, and adopt the state to drive the temperature, moisture, and mass of the tree to simulate the pyrolysis.•Use Loose Quadtree to generate tree-shape modules in real time, and tree-shape modules can be deformed by the joint action of wind field and terrain slope to control the spread of forest fire.•Calculate the barycentric coordinates of a triangle in a tree surface mesh as the position of preset flame and store in the Priority Queue to orderly generate flames on trees.