Due to the deficiencies of prior modeling methods of systems of boilers in parallel operation through a common manifold, in this article we use simplified approximate distributed modeling based on the compartmentalization and combination of the manifold. First, a principle is established to plot out the manifold into some basic pipe sections between each pair of source/sink points. Second, a simplified approximate distributed decoupled transfer function matrix model without steady errors is built for each pipe section. Finally, a smooth joint arithmetic is adopted to combine all the conterminous subsections of the pipe section with boilers/turbines at its ends into a whole system model. This method can decrease the size of the equipment and, therefore, makes it easier to model larger systems. The model presents more distributed characteristics and fits well in control system simulating. Some experiments have been done with favorable results to prove the validity of the method.