The invention provides a method of expiration control, which is used to control an expiratory valve containing a bilateral drive circuit. The method comprises the following steps: firstly, cutting off the expiratory valve before the expiration starts; secondly, opening the expiratory valve to maximum after the expiration starts; thirdly, determining the expiration time according to the sample signal; fourthly, closing the expiratory valve to a preset degree according to a certain curve linear relationship during the expiration time; and fifthly, returning to step 1.