Data acquisition in transient electromagnetic method(TEM) is in a high voltage and strong electromagnetic field. If there is not electric isolation between analog quantity and digital amount, the high voltage is easy to entry in the low voltage device and destroy it. A bipolarsignal isolation circuit was designed based on the features of the highlinearity optocoupler HCN201, and the working principle and the features of the highlinearity optocoupler HCN201 were introduced. The experimental results show the circuit has the better linearity, high acuracy and practicability in TEM system.