Abstract-The RS485 fieldbus has been widely used in communication, data collecting and related fields. The stable communication is the premise of its application. Maximum load, terminal resistor and bias resistor are three main parameters of RS485 field bus, which directly influence the stable of system. This paper adopts the current flow method to solve the theoretical value of bias resistor based on the equivalent circuit model of RS485 field bus, and further analyzes the influence on the maximum load and terminal resistor. The research result offers a theoretical basis for the application of RS485 filed bus and is very useful in many applications.  Index Terms-Fieldbus, RS485, bias resistor, terminal resistor.