The invention relates to the technical field of coffee machines and particularly relates to a bean grinding device of a coffee machine. The bean grinding device comprises a motor component, a grinder component and a transmission shaft, wherein one end of the transmission shaft is connected onto the motor component, and the other end of the transmission shaft is in contact with the grinder component so as to transfer power; the motor component is fixedly arranged in the coffee machine by a fixed ring; a spiral ring sheathes the motor component and is matched with the fixed ring so as to make the spiral ring fixed in the axial direction and rotate relative to the fixed ring in the peripheral direction; an adjusting ring fixedly sheathes the grinder component and is spirally matched with the spiral ring; and when the spiral ring rotates, the adjusting ring is driven to move axially so as to make the grinder component and the transmission shaft contact each other or separated from each other. The bean grinding device has the advantages that the structure is simple and compact and the operation is convenient.