A new optical fiber signal modulator based on interference of multi-beam is designed. In the experiment, we modulate the optical signal by modulating the distance between the two fiber-faces through the piezoelectric ceramic pipe. The modulator is operated for amplitude modulation, the modulation ratio is about 10%, S/N ratio is about 60 dB and the bandwidth is about 200 kHz. It offers a new method on modulating and can be applied to optical signal transmission in fiber and the output of optical source for fiber.