When the technique of "boiler Sultry pressure running" is applied, because boiler's efficiency raises by promoting running parameters up to the designed values, boiler's periodic running is brought forward for the high output and low load situation. By using the positive balance method, the paper is devoted to a discussion on the period running of the boiler to save energy. Relations among energy saving, boiler's running time, standing by time and time ratio are discussed in detail. In addition, the periodic running of the boiler will save the labor of the workers.