A new style tunable fiber laser in which an acoustooptic tunable filter (AOTF) and an erbium doped fiber combine to form a Fabry Perot resonator is presented and theoretically investigated in this article. It has wide tunable range and can be conveniently and rapidly tuned by changing the acoustic frequency. Under neglecting excited state absorption (ESA) and amplification of spontaneous emission (ASE), the linewidth, tunable characteristic, pump threshold and slope efficiency of the laser have been calculated. At the center wavelength of 1550 nm, an output peak with a FWHM of 0.26 nm is obtained; the tuned peak gap is about 8.95 nm when the acoustic frequency is changed 1 MHz. Theoretically, the tunable range is confined only by erbium doped fiber gain and filter bandwidth. The calculational results indicate the pump threshold and slope efficiency of this laser are 0.795 mW and 15.15%, respectively.