Abstract: To ensure the accuracy and reliability of on-board OBD (On-Board Diagnostics) acquisition terminals and improve the effectiveness of exhaust emission monitoring and overstandard screening for heavy-duty diesel vehicles, a digital metrology method is proposed. This paper introduces the definition, application, and current metrological status of on-board OBD acquisition terminals. It elaborates on the calibration method for these terminals and demonstrates their metrological characteristics from two aspects: determination of maximum permissible error and uncertainty evaluation. Three conclusions are drawn from the demonstration: the maximum permissible error of real-time engine data flow from the calibration device is set according to the data flow precision specified in the protocol, and the maximum tolerance of the on-board OBD acquisition terminal is set at twice the tolerance of the calibration device; the uncertainty introduced by the real-time engine data flow from the calibration device is U_\mathrmrel =1 \times 10^-6 , k=\sqrt 3 , and the uncertainty evaluation of the on-board OBD acquisition terminal meets the quantity transfer requirement of U\le \mathrmM\mathrmP\mathrmE\mathrmV/3 ; test results show that the indication error of the on-board OBD acquisition terminal does not exceed the maximum permissible error, the metrological performance requirements are reasonable, and the calibration method is highly operable.