In this paper, we first derive the transfer function of a broad band seismometer form its feedback model. Then, we simplify it into an approximate expression. This approximate expression is a product of a sensitivity value, a transfer function of a low pass filter and a transfer function of a high pass filter. From it, we can see how the electronic and mechanical parameters decide the seismometer’s amplitude response clearly. This information can help us to design seismometers with amplitude response that we expect. We practiced this method in the designing of EDAS-BS60 seismometer and got good consistency between theoretic amplitude response and sine calibration results of real seismometers.