Optical fiber is sensitivity to strain and bend change, so it could be used as strain sensor, curvature sensor and even weight sensor. An optical fiber weight sensor based on single-mode-multimode-single-mode (SMS) fiber structure is presented in this paper. The sensing structure is formed by cascading one segment of multimode fiber (MMF) with two segments of single mode fiber (SMF), namely SMS structure. Modal interference would occur in MMF among multiple core modes. When the axial strain and curvature on the sensing fiber structure is changed by the weight hanging at the end of the cantilever beam, the refractive index and length all change. In experiment, the weight is increased via adding different mass of weights. Afterwards, the phase difference among different core modes would be changed. Therefore, the output interference spectrum would shift accordingly. Plot the weight and wavelength shift in one graph and then it could be shown that in the range of 0 to 55g, there are three peaks shifting linearly. The weight sensitivity is up to -11.3 pm/g. This weight sensor has the advantages of low cost, simple fabrication process, electromagnetic immunity and capability of remote operation.