In order to meet the requirement of real-time estimation of the frequency of sinusoidal signals for the receiver of the reconnaissance radar,an improved Rife algorithm based on the approximate kernel and the approximate modulus method is proposed.This algorithm uses a 4-point approximate kernel Fast Fourier Transform (FFT) instead of full-precision FFT to eliminate multiplication operations,then uses an approximate modulus method to avoid multiplication operation and root extraction operation,which can reduce the calculation amount to a great extent.The simulation results show that the algorithm maintains the characteristics of high-precision frequency measurement in the full frequency band,and the Root Mean Square Error (RMSE) is close to the Cramer-Rao Lower Bound (CRLB).Even in the case that the Signal to Noise Ratio (SNR) is -7 dB,the mean value of RMSE of frequency estimation is 53.75 kHz;in the case that the SNR is 10 dB,the mean value of RMSE of frequency estimation is only 7.58 kHz.According to the analysis,the calculation amount of complex multiplication operation of the improved Rife algorithm is about 44% of that of the I-Rife algorithm.The calculation amount is greatly reduced,which can meet the requirements of real-time processing of the hardware.