A novel crystal SiGeC far infrared (FIR) sensor based on the change of resistance in a thermistor has been fabricated and developed. The developed sensor was prepared using the technology of microelectromechanical systems to achieve a better thermal isolation structure. The thermistor in the FIR detector is made Of Si0.68Ge0.31C0.01 thin films for its large activation energy and the temperature coefficient. Finite element method package ANSYS was employed to analyse the thermal isolation in the FIR detector. Responsivity, thermal conductance, thermal time constant were investigated and it was found that the thermal isolation improved structure possesses a much superior performance.