Abstract: The problems of back projection filtered(BPF) reconstruction in small field of view were analyzed, and an efficient theoretically-exact reconstruction algorithm for small field of view was proposed. With the proposed algorithm, useless computation in reconstructing PI plane was avoided by redefining the sampling method of PI plane with multi-scale method, and interpolating the two different scale PI plane data. The algorithm depressed the computation of PI plane and got the same results as original BPF algorithm, which accelerated the BPF reconstruction for solving the inutility computation of BPF algorithm in small field of view reconstruction, and also provided a good chance for clinical use.