Abstract: TDC (Time-to-Digital Converters) is very important part for flight mass spectrometer (TOF) and mars energetic particles analyzer (Mars-EPA).Furthermore,the particle identification capability in TOF and Mars-EPA is determined by TDC’s least significant bit (LSB) and accuracy.The design has used FPGA’s carry-line and coarse counter to complete TDC based on Xilinx Spartan-6.The research has designed the code-density measurement to measure delay time of carry-line delay cell.In the measurement of code density,the research finds that there is large delay time delay-cell in the carry line which will influence TDC’s extent of non-linearity and accuracy.In order to improve the accuracy of TDC,the bin-by-bin calibration has been used to calibrate non-linearity of delay line and improve accuracy of TDC.