The diagonal branch of ventilation network has the characteristic of instable airflow. It just can be seen the change of airflow direction in diagonal branch from ventilation network diagram. Ventilation network equilibrium diagram has superiorities in quantitative and intuitional reflecting the topology relation and various properties of ventilation network. Through drawing an equilibrium diagram with diagonal branch using independent path method, the instability of airflow in diagonal branch can be seen clearly. Equilibrium diagram of ventilation network provides a more effective method for studying the effects of diagonal branch on ventilation network.