Abstract: The Calibration Specification of Plate-gauge(JJG 117-2005) included in National Metrology Testing Regulation issured in 2005 is studied,in which the results given in an example of appendix A is analyzed.The results show that the loop closures induced by the errors of observation data are too large and a same measuring point has several different solutions.In this paper,in order to solve the problems,the tolerance formula of the loop closures is derived based on the surveying error theory;an improved bridge device is developed and a modified operational sequence for calibrating plate-gauge by pitch method is presented;as well as the method of least squares adjustment is adopted for observation data processing.The results of an experiment for the calibration demonstrate that the improvements made in this paper can significantly increase the level of accuracy and reliability of the calibration.