Because railway trucks bearing fault detection field conditions are extremely complicated, every electromagnetic interference signals can easily enter the measurement system along with the measured signal. To address this issue, the voltage isolation hardware circuit achieved by high linearity analog optocoupler HCNR201 and operational amplifier was designed. In the circuit, one operational amplifier is used as a negative feedback amplifier in front of the linear optocoupler, which is used to detect analog voltage signal; the operational amplifier at the end of linear optocoupler makes a conversion between the current and voltage, outputs finely voltage signal and achieves the 1:1 isolation transmission of voltage signal. The experimental results show that the measuring method can realize the good voltage linearity and high accuracy.