With the cvcrinercating differerce between wave erest and wave trough in our electric networks, some large-scale fire-eleciricity units which were on the basic load are now involved in the crest modulation. In the present paper, the authors lake the 300MW imported boiler unit in Yaomeng Power Plant as an example and make a study and analysis of the variable pressure's runningability and thecharacteristics, of crest modulation.