Abstract: The neutron count rate of detector in fastthermal boundary showed quite different performance in critical extrapolation experiment on Venus 1#, which was listed as a benchmark of accelerator driven subcritical system (ADS). In order to explain the abnormal phenomenon in experiment, numerical simulations of experiment and calculations of neutron spectrum in fastthermal boundary were performed, analyses to the abnormal neutron count rate were also represented through calculations. The results indicate that neutron spectrum change during critical extrapolation is the main contributor to the abnormal performance of detector in experiment. This research work will supply theoretical basis for neutronics study on fastthermal coupling subcritical systems of the future.