In the broadband communication network, the wavelength-division-multiplexed (WDM) system is widely used to maximize the information that the signals can carry. As a result, the number of channels which are carried by different optical wavelengths in the WDM optical fiber network also keeps increasing. To separate the huge number of different wavelength signals, optical filter is required. The optical filter based on semiconductor has been widely studied due to the maturation of semiconductor fabrication technology and that it is possible to integrate the filter with the stable semiconductor devices such as laser diodes and MOSFETS. The tunable optical filter is basically a selective optical resonator that only allows the resonant modes passing through. Various mechanical methods are studied to achieve the tunable effect by tuning the physical structure of the filter; however, there is not much research on how the semiconductor material will affect the tuning function. In this paper, the author studied the influence of refractive index of the multi-silicon-slabs on the filter, whereby the tuning of refractive index is reached by thermal effect. It is found by simulation that when heating the silicon slabs, the increasing refractive index of silicon will lead to a shift of the resonant mode wavelength. This shift is almost linear with the change of the temperature, which is about 1nm with every 20K temperature increase. For certain devices, the result of the simulation showed it is possible to tune the resonant mode from C band to L band in the Fiber Optical Communication.