This paper deals with the analysis of a single degree of freedom vibration isolation system with dry friction damper. Based on the analysis of classical vibration isolation transmission ratio, the conditions of free resonance are found. Regard to the character of vibration isolation system on critical frequency of vibration isolation and resonance, the transmission ratio formula of vibration isolation with friction force under full frequency range is educed which provided the rounded theory principle for free resonance vibration isolation. The vibration isolation system is designed which can realize the non-amplification in low frequency range and better vibration isolation effect in high frequency. Lately, the correctness of the principle is verified by the actual experiment.