Abstract: During checking continuous waveform records, it was found that some stations of Tianjin such as EWZ were significantly affected by one unknown noise, and the EWZ station is most affected. Long term observations have also found that this noise cannot be observed for several days each year, but the timing is uncertain. This noise is all year round and does not change with day and night time, and it has been existed for many years. This noise is different from typical environmental background noise and has some fixed characteristics. This article studied the noise from several aspects such as its own characteristic, its coverage of influence, spectral characteristics, and impact on earthquake records. The research has shown that the noise is not affected by natural factors such as weather and season, and has a characteristic of amplitude decreasing as the distance increasing from EWZ station. The main frequency range of this noise is 1～2 Hz. The impact range of noise is relatively wide and it can be recorded by 12 Surrounding stations, with YGZ station being the farthest and approximately 58 km away from EWZ station. Through polarization analysis, it was found that the propagation of noise has a specific directionality, so it is preliminary judged that this noise has the characteristic of fixed source noise. This noise will have an impact on seismic records, reducing the seismic phase pickup rate, and even completely submerging it in the noise. Frequency filtering will improve it.