The multiple-beam interference of the echo wave which is aroused by the Fresnel reflection between two end faces of a couple of fibers is analyzed theoretically and the intensity of echo wave at the end face of fiber is calculated. A new modulator for signal in fiber is designed based on the echo interference. In the experiment, the optical signal is input to the inputend of the coupler,and the modulating voltage signal is amplified and drives the piezoelectric ceramic pipe vibrating in order to control the distance between the two fiber end faces. At the same time the light can be reflected from the two faces of the fibers and the interfered echo wave signal can be modlated. In the experiment,the device is operated for amplitude modulation,the modulation ratio can reach 95%. the S/N ratio is about 10 dB,and the bandwidth is about 200 kHz. This device is innovative,can be used easily and has low price,and can be applied as a modulator or chopper in optical signal transmission system.