With ultra-high index contrast and ultra-small cross sections, silicon nanometer optical waveguides have super light field limit ability and provide a very promising way to realize nano-photonic integrated circuits with high integration density. It is well-known that mode conversion and coupling play an important role for realizing various functionality elements in photonic integrated circuits. The theory of mode conversion and coupling new structures and devices in silicon photonics integrated circuits are analyzed and discussed in detail. The mode transmission and evolution process of silicon nanometer optical waveguide tapered structure are studied, and the unique polarization-dependent mode conversion mechanism is revealed. The results show that when asymmetry exists in the cross section of the optical waveguide, it is possible to produce polarization mode miscellaneous in some specific waveguide widths. Which provide a convenient method for realizing polarization rotation. By adjusting the phase matching conditions of mode conversion and coupling in asymmetric directional coupled structures, important approaches for realizing ultra-small polarization-beam splitters as well as broadband mode multiplexers/demultiplexers are provided.