A ring-cavity fiber laser using a 973-nm laser diode (LD) as pump source and a highly Yb³⁺-doped fiber as gain medium is built. A laser output with a continuously-tunable width of 24 nm (1030-1054 nm) and a threshold power of 49 mW is obtained by tuning the intra-cavity polarization filter,which is made of a common fiber and some polarization elements. Moreover,we significantly expand the tunable range to 57 nm (1021-1078 nm) by introducing uniform stress in a part of cavity fiber. Compared with other tunable Yb³⁺-doped fiber lasers on the principle of polarization filter,this laser system has realized a much wider tunable width.