A sinusoidal phase modulator composed of a piece of single-mode fiber around a piezoelectric transducer is inserted in the linear resonant cavity, which makes the loss corresponding to different lasing lines uncertain, the mode competition owing to the homogeneous broaden line in erbium-doped fiber is suppressed and the unstable single-wavelength lasing at room temperature is eliminated. In order to achieve flat multi-wavelength output, a long-period fiber grating with loss peak at 1530 nm is used to obtain flat gain spectrum. The stable output from the multi-wavelength erbium-doped fiber laser is realized. Simultaneous multi-wavelength lasing is observed with 0.45 nm intervals defined by the reflective comb-like filter. In the meantime, there are 9 lasing lines in power differences less than 10 dB and the side-mode suppression ratio is higher than 25 dB.