Vertical cavity surface emitting laser (VCSEL) is widely used in the interconnection of new-generation computer, which is a multi-mode laser with the wavelength at 850 nm. A rib waveguide with section area similar to the output plane needs to be developed to fit VCSEL better. Based on the successful preparation of polysiloxane rib waveguide, which has the smallest area of 70 μm×50 μm and straight length of 21 cm, theoretical analysis of its loss mechanism is processed. By using perturbation theory, it is concluded that the absorption of material can be neglected and the main loss of scattering comes from the loss of surface scattering induced by the roughness of top surface, bottom surface and side surface. By using variational principle and finite element method, the field distribution influenced by bubble, defect and impurity is analyzed. Concrete measured value of bubble, defect and impurity at saltus, visible influence of basic mode and first-order mode is gotten. Transmission loss measured by CCD photography shows that experimental results fit the theoretical results well.