Aiming at the problem that of low machining accuracy of a certain type of small five-axis high-precision CNC machine tool is not high， the dynamics of its beam is studied. The natural frequency of the beam is studied by combining the hammering mode test experiment and finite element simulation calculation. Measure the vibration speed and vibration noise at different speeds of the spindle， and the vibration evaluation index equation is established through weighting processing to obtain the optimization target. The shape of the studied beam is optimized. The maximum error of the natural frequency obtained by the test and simulation is 5.76%， and the frequency range of 83~117 Hz is determined based on the spindle vibration， which is the low-order natural frequency range that needs to be avoided in the design of the beam. After shape optimization， the first-order natural frequency of the beam is 139 Hz， which is an increase of about 20.870% compared with the original beam， and the dynamics of the beam is improved.