The invention provides a modification method and device for a target roll gap value of a withdrawal and straightening machine. The modification method for the target roll gap value of the withdrawal and straightening machine comprises the steps that a pulling-straightening roller of the withdrawal and straightening machine is pressed on a calibrating position to obtain reading Li of a corresponding displacement sensor during the process of dummy ingot delivery; a calibration pressure F0 of each withdrawal and straightening machine and an original reading L0 of the displacement sensor are set as a standard, and a modification value delta i=Li-LO of the roll gap of each withdrawal and straightening machine is obtained; pressure Fj of the withdrawal and straightening machine executing pressing is sensed through a pressure sensor; whether Fi equaling to Fj is existed or not is judged; if Fi equaling to Fj is existed, modification is conducted on the target roll gap value through the roll gap modification value delta i, and the target roll gap value = a standard roll gap value-rolling reduction+the roll gap modification value; if Fi equaling to Fj is not existed, a pressure section in which Fj is located is found out, a modification value delta j corresponding to the pressure Fj is obtained through interpolation, and modification is conducted on the target roll gap value. According to the modification method and device for the target roll gap value of the withdrawal and straightening machine, modification is conducted on the target roll gap value of the withdrawal and straightening machine executing pressing through the roll gap modification values corresponding to different pressures, and the control accuracy of the roll gap is improved.