A method for online calibration of a width gauge belongs to the technical field of metrology and calibration methods, and is used for on-line accurate calibration of width gauges. The technical solution is that the method comprises the following steps of determining the relationship between the quantity provided by the measurement standard and a corresponding indication, determining the relationship of a measurement result obtained by the indication by using the information, and sequentially performing calibration measurement, calculation of an indication value error of the width gauge, calculation of repeatability of the indication, calculation of indication drift, data processing of calibration methods, and evaluation of standard uncertainty. The method provided by the invention, which is a major breakthrough in the online calibration problems of large equipment in the metallurgical industry, uses the error theory to design the standard equipment, determines the measurement requirements of the product, uses statistical methods to check the consistency of the measurement results, evaluates the measurement uncertainty of the measurement results and proves that the requirement meeting of quantity transfer and calibration development. Compared with the traditional inspection calibration method, the method provided by the invention saves a lot of time and labor costs, and is morescientific, active, reliable, accurate and easier to implement.