Abstract: The timing accuracy of the timing system directly affects the final flow measurement result of liquid flow standard devices, which requires regular maintenance and calibration. The traditional timing verification system is often bulky and difficult to carry and transport. It is generally integrated with the measurement and control system, difficult to disassemble and send for inspection, and lacks the calibration interface suitable for the experimental characteristics of flow standard device. In view of the limitations of the existing timing system verification methods, a portable time interval measuring instrument was designed and developed for the instantaneous flow method liquid flow standard devices, which can measure the time interval range from 1 μs to 10000 s, with measurement accuracy of 10 μs, display resolution not less than 1μs, and the accuracy is not less than 1.0×10⁻⁶. The performance verification experiment was carried out based on the hot water flow standard facility of China National Institute of Metrology, and the test results showed that: The relative error of the timing system is better than 0.0024 %, and the relative error of the time interval measuring instrument is better than 4.2×10⁻⁷ and its repeatability is better than 1.82×10⁻⁷. 10 μs, the display resolution is not less than 1μs, and the accuracy is not less than 1.0×10⁻⁶. The performance verification experiment was carried out based on the hot water flow standard facility of China National Institute of Metrology, and the test results showed that: The relative error of the timing system is better than 0.0024%, and the relative error of the time interval measuring instrument is better than 4.2×10⁻⁷ and its repeatability is better than 1.82×10⁻⁷. Its timing accuracy is high and its performance is stable and reliable, which can effectively meet the verification and testing requirements of the timing system of the liquid flow standard facility. It also plays a positive role in improving the measuring capacity of liquid flow facility and the development of liquid flow measurement technology in the future.