A low frequency vibration-isolating system was designed in order to meet the special requirement of vibration isolation of Raman laser reflector in atom interferometers. The system uses both two-step passive and one-step active isolations. Analysis of the mechanical structure and the feedback loop was carried out in detail. The performance of the system using Matlab Simulink software was assessed, and the result shows that it has an isolation ratio up to more than 99% for 0.1～10 Hz frequency range.