The invention provides a method for purifying propylene oxide, and relates to a method for separating methanol, methyl formate and aldehyde in the propylene oxide through extracting and rectifying by continuously operated reactions, wherein, the aldehyde can be formaldehyde, acetaldehyde and propionaldehyde. The method comprises the following steps of: adding an alkaline solution containing ammonia or aminium salt above a feeding point of crude propylene oxide; and a reflux ratio of a condensate liquid selected to return a reactive rectification tower to the purified propylene oxide as a product is between 1 and 5, thereby the content of the methanol in the propylene oxide is less than 600ppm, the content of the methyl formate is less than 100ppm, and the content of the aldehyde is less than 100ppm.