Abstract: Measured the first six-order natural frequencies of the shell of counter-rotating axial flow fan by modal test, the fourth harmonic of the fundamental frequency of fan was similar to the fifth-order natural frequency of shell, so we should optimize the structure of fan to avoid resonance. When the fan operated at the highest point of efficiency at the rated condition, measured the vibration of different directions in different positions. Results show that, the vibration at inlet and outlet of the fan is small, and the frequency is mainly doubling of the base frequency of the fan. The vibration at impellers and motor is large, and the high wide-band frequency is mainly caused by the aerodynamic force of the flow field. The vibration of the fan's top horizontal direction is the most severe, which can be reduced by adding a damping device at the top.