We introduce a non-standard finite difference method to approximate solutions of a class of nonlinear reaction-diffusion equations with variable coefficient. These equations describe the  growth, decay, colony and mutual transformation processes of biomass. So, these solutions of model must be bounded and non-negative. Due to obtain the exact solutions is very difficult task, whence, it is high significant that  designing numerical techniques to approximate them. Our FDM are quite efficient for ensuring that their boundedness and non-negativity. We All these examples have showed that our non-standard FDM can ensure the solution automatically bounded, as long as some  conditions are satisfied.