As a new kind of flowmeter, A+K balance flowmeter was invented for the oxygen measurement of space shuttle's main engine. Compared with other differential pressure flowmeters, it has many advantages such as short required straight pipe, high precision, low pressure loss, wide applicability, etc. Aiming at its feature of short required straight pipe, this paper studies four kinds of flowmeters, i.e., DN50, 0.25 and 0.70 equivalent diameter ratios and DN100, 0.25 and 0.65 equivalent diameter ratios, respectively. The minimum required length of straight pipe was studied through experiments when double 90° elbows on the same plane are installed in the upstream. Finally, the minimum required straight pipe length between A+K balance flowmeter and double elbows are given by means of the qualitative and quantitative analysis.