The invention provides a focusing method and device for a laser marking machine and the laser marking machine. The focusing method for the laser marking machine comprises the steps that a marking head outputs an optical signal to each optical signal receiver; each optical signal receiver converts the received optical signal into a corresponding electrical signal value; each electrical signal value is compared with a prestored optimum focal distance electrical signal value, and a minimum difference value electrical signal value is selected; a prestored height value of the optical signal receiver corresponding to the minimum difference value electrical signal value is acquired; and the marking head is adjusted to move by the prestored height value. According to the focusing method and device for the laser marking machine and the laser marking machine, the invisibility of laser is overcome, the invisible laser is converted into the quantifiable electrical signal values, moreover, the electrical signal values are compared with the optimum focal distance electrical signal value, so that the value of the height difference between a focus and a marking plane is obtained, and the marking head can be directly adjusted according to the value of the height difference; and focusing operation is simple, focusing is accurate, the consistency is high, and the marking effect of the laser marking machine is enhanced greatly.