Using a 355 nm CW laser to pump BBO crystal, the measurement device was set up based on spontaneous parametric down conversion. The channel detection efficiency of system at 737 nm was calibrated at different photon rates, with time-to-digital conversion and time-to-amplitude conversion for performing coincidence counts. The influence of death time and afterpulse on the selection of coincidence gate width and accidental coincidence at high photon counting rate is analyzed. The measurement results of different photon counting rates are compared and the correction factors of coincidence measurement are analyzed. The experimental results show that the relative deviations of the two coincidence measurement methods are better than 0.25%. The afterpulse probability at different photon counting rates are measured by time interval analysis, which provides a basis for improving the accuracy of single photon detectors counting and entangled photons photon calibration.