The measurement accuracy of terahertz frequency by using the frequency comb method depends on the locking accuracy of the repetition rate and the measurement accuracy of the beat signal. The frequency of the beat signal is measured by the frequency counter, so the signal-to-noise ratio (SNR) and the power of the beat signal are required to be high enough. The higher the SNR, the higher the accuracy of the frequency measurement. Therefore, the detection of the beat signal and high SNR are very important in terahertz frequency measurement. The main factors of the SNR of the beat signal are studied systematically, including the generation method of the beat signal, the signal amplification, the beat frequency and the power of the measured terahertz source. Through comprehensive optimization, the beat signal with SNR better than 60 dB is achieved experimentally, which provides a good foundation for the high accuracy measurement of terahertz frequency.