Compares with single circuit control system on the structure and composition ,the cascade control system has a measurement unit, a controlling unit and a closed circuit. Measure the unit to get physical quantities as auxiliary regulated variable for the system, and take the regulating unit added to be the auxiliary governor, and add a small closed circuit as auxiliary circuit of the system. The adjustment function of the cascade control system is still to make the regulated variable equal to the set value. There is still only one actuator and one regulating mechanism. For those complex object being strict with performance indexes of automatic adjustment, but the single circuit control system can not meet the requirements, and it needs to consider to use cascade control system.