The mixed compressor has advantages of high pressure ratio of single stage, large flow rate and high efficiency of axial flow compressor. On this basis, a single-stage mixed flow compressor with a speed of 40 000 r/min,pressure ratio of 3.5 and flow rate of 3.2 kg/s was designed for a micro turbojet engine. In order to study the flow mechanism of the compressor and the range of stable operation, the operating conditions of 90%,100% and 110% of the design speed were analyzed respectively, and the flow fields at near stall point, near plug point and the highest efficiency point at off-design rotational speed were analyzed emphatically. The results show that the design point simulation pressure ratio is 3.662, flow rate is 3.324 kg/s, entropic efficiency is 87.61%, indicating that the designed compressor is reliable;the main factors leading to compressor instability are shock loss of blade pressure surface, suction surface and low energy fluid at the tail edge of suction surface.