A new kind of piezoelectric actuator for low frequency isolation has been designed aiming at the low frequency disturbance of a 6 DOF platform. An actuator dynamic experiment system has been set up to analyze the relationship between the input voltage and the output force.The experimental results show that the input output of actuator does not match with piezoelectric effect when the actuator is driven by a motional voltage of less than 25 Hz.Based on the phenomenon,a double polyline model suitable for low frequency control actuator of platform has been established.The test results show that the low frequency model has a good agreement with the actual characteristic of the actuator.