Abstract: Nuclear reactor instrumentation system measures the nuclear pulse signal output by the detector. The trailing edge of the signal is very long, and the problems such as signal accumulation and baseline drift are likely to occur when the counting rate is high. Thus, the upper limit of the source range counting rate can only reach 105 Hz. Based on numerical differentiation method, this paper analyzes the output of the signal after preamplification, the signal shaping, the low-pass filtering and pulse discrimination in the time domain by digital processing technology, and the simulation is carried out based on the signal measured by the detector.  Simulation results show that, the digital processing method based on numerical differentiation can realize the identification and measurement of adjacent 0.4 μs pulse signals, and raise the upper limit of the source range counting rate to 2×106 Hz or more.