The invention relates to the field of double-bypass compression system tests, in particular to an adjusting and controlling device and method for the bypass ratio of a double-bypass compression system. The adjusting and controlling device for the bypass ratio of the double-bypass compression system comprises a double-bypass compression system test piece (4) and an outer bypass exhaust system, oneend of an outer bypass gas collecting cavity (71) of the outer bypass exhaust system is connected with the double-bypass compression system test piece (4), and the other end of the outer bypass gas collecting cavity (71) of the outer bypass exhaust system is connected with an adsorption pipeline (72); and the adsorption pipeline (72) is sequentially provided with a Venturi nozzle (8), a bypass exhaust adjusting valve (9) and a cooler (10) and finally connected with a water ring vacuum pump (11). According to the adjusting and controlling device, the bypass ratio is adjusted in a double-bypasscompression system test through the outer bypass exhaust system provided with the water ring vacuum pump, the defect that as for a traditional conventional test method, the autonomous exhaust capacityof an outer bypass is low is overcome, discharge and flow control over outer bypass airflow can be achieved, and thus the bypass ratio of the double-bypass compression system is adjusted.