Physical mechanism of using nonlinear polarization rotation as saturable absorber to obtain pulse is analyzed theoretically.Experimental study of improved femtosecond fiber ring laser is reported. A fiber ring laser with highly Er3+-doped fiber as gain medium is demonstrated, 976 nm laser diode (LD) laser is used as pump source, and in the fiber ring cavity, nolinear polarization rotation mode-locking is realized by adopting single polarization controller. By tuning polarization controller , stable mode-locked pulses centered at 1535.9 nm with 23.5 nm bandwidth at 3 dB, 131 fs duration and an average output power as high as 5.91 mW are successfully obtained at the repetition rate of 11.20 MHz.