Abstract: Physical Unclonable Functions (PUF) is a new method for the safety of Integrated Circuit (IC) products. Nowadays, the Ring Oscillator (RO) based puf is under the shadow of being attacked because of RO's constant frequency. A new construct(Multiple Frequency Slots based PUF,MFS-PUF) is proposed to solve this problem. All the ROs are configurable, each RO's frequency changes from one to another after generating one response bit. In each frequency, ROs differ from each other because of the uncontrollable difference in manufacture and there exists a frequency slot. In the whole system, many frequency slots exist and the transfer between them are unpredictable, this makes us get more Challenge-Response Pairs (CRPs for short), what's more, the system is more unpredictable. Compared with the traditional RO based PUF, it is more difficult for the attacker to model this system. This architecture not only ensures the uniqueness, but also increases the safety of itself.