Abstract: In order to achieve high-precision and high-resolution measurement of γ-γ coincidence time in the heavy-ion cancer therapy device, a newly implemented coincident time measurement system is designed. The system is mainly composed of a timing discrimination circuit constructed by the high-speed comparator and FPGA-based time-to-digital converter (TDC). A precision time measurement is performed by TDC through the combination of coarse time and fine time measurement methods. The coarse time measurement is realized depending on a binary counter, and the fine time measurement is realized by using the cascade delay chain developed with the in-chip buffer unit and register array. The measured results show that the intrinsic time interval measurement resolution is better than 276 ps (FWHM). The system were tested combining with a coincidence detecting system built by H8500 coupled to LYSO scintillators arrays and the pairs of γ-rays with 511 keV from ²²Na source. The results show that the time accuracy of coincidence detecting system is better than 1.12 ns (FWHM) and the black scattered points on LYSO crystal flood image are clear and easy to be distinguished.