The advanced co-generation system (ACGS) was built at Waseda University in 1999. The ACGS consists of a gas turbine, a steam turbine, an ammonia-water mixture turbine, an ammonia absorption refrigerator and an ice storage system. These turbine systems have a cascade relation upon the utilization of energy. Meanwhile, the ammonia-water mixture turbine and the ammonia absorption refrigerator located in the bottoming stage of the ACGS to utilize the lower temperature steam. In this paper, a mathematical model of the bottoming stage of the ACGS is established according to the experimental data. And, the operational planning of the bottoming stage with three cases of the cooling demand during the nighttime is also discussed.