Seismic noise is one of the crucial problems for terrestrial precision experiment.It can not be measured by the general interferometric method because the seismic noise usually has the level of nanometer.A laser in interferometer control system,which consists of a laser interferometer,a detector,a control circuit,and a displacement feedback actuator (piezoelectric ceramic),has been built up.The low-frequency (0.3~3 Hz) seismic noise level is measured using the lock-in laser interferometer combined with a long period folded pendulum.The results show that the maximum vibration noise level is 10-9m/Hz at 1 Hz.