The invention provides an automatic knife changing method which comprises the following steps: a knife handle assembly loaded with a knife is grabbed through a knife grabbing mechanism; the directionof the knife handle assembly is changed through a grabbing and rotating air cylinder, and then the knife handle assembly is conveyed through an x-axis linear module to the position of a knife handle grabbing assembly; the upper-lower position is adjusted through a telescopic air cylinder, and then the inserting direction of the knife handle assembly is adjusted through a fine tuning and rotating air cylinder and the knife handle assembly is inserted into the knife handle grabbing assembly; the knife changing process is driven by a drive motor; a drive part A and a drive part B are driven through a drive incoming block and a drive transferring block to rotate; then, a drive rotating body and an ejection rotating body are driven to rotate, a secondary drive block is ejected and driven by thedrive rotating body through an ejection drive table, and a knife handle clamping plate is driven by the secondary drive block through a secondary grabbing swing arm to loosen a knife handle clampinggroove; and at the moment, an ejection bearing moves on an ejection and drive translation table and then rotates on the side surface of an ejection and drive inclined groove, so as to eject a knife handle body.