Abstract: The standard gas flowmeter with the lower limit of 5×10^-16 Pa·m³/s is designed to solve the traceability of measurement value for ultra-sensitive leak detector.The flowmeter is designed based on the constant conductance method, and the range of it is decided by the conductance of the molecular flow element and the inlet pressure of the element.The molecular flow element with the conductance of 10^-10 m³/s can be achieved by laser drilling on a platinum sheet and vacuum coating process, and the inlet partial pressure range of helium from 10³ to 10^-6 Pa can be reached by combining three methods, which are the direct gas supply method, the expansion method, and the mixed gas preparation method.The standard gas flowmeter can provide gas flow with the range of(10^-5-10^-16) Pa·m³/s only using the capacitance diaphragm gauge as reference pressure standard and two molecular flow elements with the conductance of 10^-8 m³/s and 10^-10 m³/s.And the combined standard uncertainty of the flowmeter is estimated to be 1.2% to 2.8%.