We demonstrate a tunable multiwavelength fiber laser in this paper. The multiwavelength comb is produced by a self-seeded Brillouin-erbium fiber laser (BEFL). The self-seeded Brilllouin pump is achieved by incorporation of a transmissive Sagnac loop filter and 2 km single mode fiber. We study the influence of the transmissive Sagnac loop filter on the BEFL in detail, include the tunable characteristics of the BEFL, and the influence of the Sagnac loop bandwidth on the multiwavelength band. A multiwavelength band with 52 lines is achieved when the bandwidth of the Sagnac loop filter is 83.3 nm. At the same time, we observed dual Brillouin bands exciting.