Abstract: With some known typical seismic data as reference, we analyzed the essential differences between strong motion observation and small earthquake observation, and compared the electric noises of FBS-3B seismometer and BBAS-2 accelerator. We carried out a synthetic simulation using the data of a small earthquake recorded by Xiaowan network. The result reveals the influence of electric noise and response type of the sensors on their sensitivities to ground motion. With asynchronous synthetic sine waves as input of a pendulum response equation, the output of a seismometer and accelerator is obtained. The sensitivities of the sensors to a teleseismic wave prove to be quite different and it is this difference that leads to the distinction of recorded data.