It is found that the Er3+ doped fiber laser is in pulsed state when it is phase modulated at a low frequency. A pigtailed LiNbO3 phase modulator driven by a commercial signal generator was inserted into the fiber laser cavity, regular pulses were achieved in certain conditions and other phenomena such as the change of the laser spectrum were observed. The pulse had a repetition rate from several tens of kHz to several hundreds of kHz. Analysis results have shown that it is hard to be explained by the traditional theories such as Q-switched and mode-locking theory. The mechanism of these phenomena is discussed briefly. Based on the investigation of the relationship between phase modulation and relaxation oscillation, the phase modulation and relaxation oscillation are phenomenologically considered as the reason of the pulsed laser.