Abstract: During checking continuous waveform record, it was found that some stations of Tianjin such as EWZ were significantly affected by one unknown noise, and EWZ station was the most seriously affected. Long term observations had also found that this noise cannot be observed for uncertainly several days each year. This noise was all year round and did not change with day and night time, and it had been existed for many years. This noise was different from typical environmental background noise and had 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 was not affected by natural factors such as weather and season, and had a characteristic of amplitude decreasing as the distance increasing from EWZ station. The main frequency of this noise ranged 1～2 Hz. The impact of noise was 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 had a specific directionality, so it is preliminary judged that this noise had 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 can improve it.