A carrier frequency offset estimating device for estimating frequency offsets of a multi-carrier signal includes a transforming circuit, a cosine effect eliminating circuit, a model creating circuit, and an estimating circuit. The transforming circuit receives the multi-carrier signal and transforms the multi-carrier signal to single-carrier signals. The cosine effect eliminating circuit eliminates cosine effects in the single-carrier signals to form single-carrier signals without the cosine effects. The model creating circuit creates linear data models according to the single-carrier signals without the cosine effects. The estimating circuit estimates the frequency offsets of the multi-carrier signal according to the linear data models. A carrier frequency offset estimating system is also provided.