The configuration of passively mode-locked fiber laser is simplified by introducing an in-line rotatable in-line polarizer into the ring cavity.By adjusting the angle between the polarizer and the 1/4 wave-plate,the passively mode-locked laser is obtained.Based on the simple configuration,an automatically passively mode-locked fiber laser is realized by designing the microcomputer circuits to control the rotation states of the polarizer and the wave-plate.A mode-locked laser with a repetition rate of 5.6 MHz and average output power of 8 mW are obtained.