The heat supply retrofitting is carried out on existing 300 MW sub-critical direct air cooling units to meet the increasing heating demand in northern China. Two waste heat utilization technologies for power plants were introduced, namely high back pressure heat supply and absorption heat pump heat supply. Based on the economic analysis of two waste heat utilization schemes, the proper solution was determined for the direct air-cooling power unit. The results show that high back pressure heat supply and absorption heat pump heat supply both have remarkable energy-saving effect, decreasing the total coal consumption. For given load, the coal rate of high back pressure heat supply is even lower than that of absorption heat pump heating with the maximum heating area and heating load. Specifically, under the maximum heating load, the coal rate is reduced to 168.9 g/(kW·h) and 197.2 g/(kW·h) through high back pressure and absorption heat pump heat supply. Under the same power load, the maximum heating load and heating area of high back pressure heat supply are 73.6 MW and 133.8×10⁴ m² higher than those of absorption heat pump heating.