During altering the ordinary machine tool (CW6100) to the special pressure roller grinding machine, we applied a numeral alteration method in which the PLC was the kernel part because we had to take into account the factor of cost and workload. The method only added a motional device in X-axis of the machine tool and achieved the harmony of the continuous motion of y-axis and the digit-controlled motion of x-axis. The experiments showed that the method was simple and the cost was cheap. The machine tool could machining the long pressure roller.