The model of collision on safe cab of farm foresty vehicles has multi nonlinear features on geometry and material. The large scale deformation elastic and plastic and plastic theories are applied by setting up the mathematical model for cab, the strength feature of it is analysed. The relation of force and deformation when vehicle rolled is predicted. The safety of driver's space ingressed by deformation of components of the cab is estimated, and the deformation failure is assessed. The numerical simulation for collision on safe cab of vehicle is realized. Based on a type of farm foresty vehicle, simulations based on theory and experiment are conducted. Both results are basically coincident. This simulation model is practical and feasible, which can provide reference for safety design of vehicle cab.