The modified polydimethylsiloxane (M-PDMS) is synthesized by adulterating cross-linking of polysiloxane (HPSO) and divinylbenzene (DVB) into polydimethylsiloxane (PDMS). M-PDMS refractive index contrast PDMS are changed by 1%~1.8%. PDMS and M-PDMS bulk samples have well transmittance and low propagation loss. These benefits realize high productivity and low-cost mass production of large core multimode optical waveguides for optical interconnection. M-PDMS and PDMS are used for the core layer and the cladding layer. Large core(50 μm×50 μm) multimode optical waveguides are fabricated by the soft molding and simple replication, the length of waveguides is more than 20 cm. The optical propagation loss of waveguides is 0.137 dB/cm measured by digital scattering method at 633 nm.