A new type of Si waveguide wrapped by silicon nitride (SiN) is designed, and its optical and thermal analysis are presented. The thickness of SiN up-cladding should be larger than 1 μm in order to prevent the absorption of optical field by metal heater. Thermal response of the proposed waveguide structure is enhanced by the high thermal conductivity of SiN. Moreover, this thermal response can be further improved by a fast heat dissipation channel created in this structure. Our simulation results indicate that a rise time of about 110 ns can be achieved for the proposed waveguide structure, which is about two orders of magnitude less than that of the conventional Si waveguide. The influences of the thickness of up-cladding and the stretching width and etching depth on the thermal performance are also discussed. The simulation shows thin up-cladding, large stretching width and etching depth are critical to enhance the thermal response speed.