A Fabry-Perot self-collimation interferometer (FPSI) constructed in a two-dimensional photonic crystal (2D PhC) by liquid crystal infiltration has been proposed and demonstrated theoretically. The resonant cavity of FPSI is infiltrated with a nematic liquid crystal (LC) 5CB with ordinary and extraordinary refractive indices 1.522 and 1.706, respectively. The transmission spectrum of the FPSI has been investigated with the 2D finite-difference time-domain method. Calculation results show that resonant transmission peaks have nearly equal frequency spacing 0.0090c/a. When the effective refractive index neff of the liquid crystal is increased from 1.522 to 1.706, the peaks shift to the lower frequencies over 0.0071c/a while the peak spacing is almost kept unchanged. Thus this FPSI by LC infiltration can work as a tunable attenuator or an optical switch. For the central operating wavelength around 1.55m, its dimensions are only about tens of microns. Thus this device may be applied to photonic integrated circuits.