Abstract: With the development of protected agriculture, the size of environment control system is larger and larger. The RS-485 bus has the virtues of simpleness, convenience to use, appropriate distance of communication, proper rate of transmission and high reliability. It is widely used in the control of agricultural environment. But TIA EIA-485 stipulates that an RS-485 connection has no more than 32 noads. To solve the problem of more than 32 nodes in the communication network, a method of RS-485 bus topology based on single chip is put forward in the paper. The method adds the capacity of RS-485 bus. The bus network can extend to four, eight and twelve branches according to the concrete case. The design is simple and the cost is low. The circuit runs smoothly and reliably.