The Straw-Tube-Tracker (STT) is an essential detector in the COSY-TOF experiment which provides accurate position information of charged particle tracks to study strangeness physics. The STT consists of 2704 straw tubes in 13 double-layers, installed in a vacuum tank behind a liquid hydrogen target. The spatial resolution of 150 and an efficiency of 98% has been achieved under experimental condition for the individual straws. The STT performance is crucial for geometrical track reconstruction and could be improved further by optimizing the calibration which will increase the reconstruction efficiency and resolution. The new calibration has included different systematic effect corrections e.g. electronics offset and signal width. Hence, pp elastic scattering events measured in fall 2012 at 2.95 GeV/c beam momentum were analyzed for the improved calibration of the STT. The spatial resolution of the COSY-TOF STT could be improved by about 20% with the new calibration.