The frequency of ground records on liquefied and non-liquefied sites is different. The calculation method of frequency decreasing rate is given here and the division line between liquefied and non-liquefied sites is proposed. To analyze the time-frequency curves of acceleration, the zero-crossing method is employed. The soft sites and ordinary non-liquefied sites are included in non-liquefied sites. The results show: (1) The concept and calculation method of frequency decreasing ratio, which is proposed in this paper, can describe the characteristics and regulations of time-frequency on liquefied and non-liquefied sites; (2) Before peak ground acceleration (PGA), the difference of the average frequencies of acceleration on liquefied and non-liquefied sites is not obvious and the average frequency of acceleration on soft sites is smaller than that either on liquefied or non-liquefied sites; (3) After PGA, the average frequency of acceleration on ordinary non-liquefied sites is the highest in the three types of sites, that of soft sites is the middle and that of liquefied sites is the smallest; (4) If the absolute change of the time-frequency is used as the criteria, it will be confused between soft sites and liquefied sites; (5) The threshold value of frequency decreasing ratio is 0.5 between liquefied and non-liquefied sites, which can judge the liquefied sites, non-liquefied sites and soft sites correctly.