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A new mass-time primary standard for high pressure natural gas, which is based on electromagnetic balance and hydraulic fast-acting valves, was set up at the beginning of 2017 in Chengdu, China. The full load of the electromagnetic balance is 3 tons and the measurement uncertainty of mass is better than 1.0g(k = 2). The opening and closing time of the hydraulic fast-acting valves can achieve 33 ms±3 ms.The operation pressure and flowrate range of the facility is (4–60)bar.a and (5–410)m3/h respectively. In accordance with the preliminary tests, the estimate uncertainty of sonic nozzles calibration is between 0.10% and 0.12%(k = 2). The operation principle, testing results and the uncertainty evaluation are presented in the paper as well as some improving ideas. • A new mass-time primary standard for natural gas based on 3t electromagnetic balance and hydraulic fast-acting valves. • Gas collecting tank, balancing tank and weights are operated automatically by PLC. • Two hydraulically-actuated valves equipped with 13 optical gratings are operated in about 33ms and "zero overlap" are achieved. • Uncertainty evaluation and one year time stability test results show that the CMC is about 0.10%-0.12%(k =2). • Inter-lab comparisons with NMI and CEESI also verify the CMC. [ABSTRACT FROM AUTHOR]