In the greenhouse environmental monitoring system, the wired sensor networks have some issues, for example, complicated wiring, inflexible sensor location, cable soften and corrosion and so on. To solve the above problems, we use ZigBee technology to build a wireless sensor network for monitoring temperature, humidity, light intensity, carbon dioxide concentration. And each node is low-power design. Based on the Modbus protocol, we built RS-485 bus to achieve the communication between a number of greenhouse sensor networks and upper computer (PC). The system has advantages such as flexible sensors placement, low power consumption, easy installation maintenance and expansion, low cost, strong practicability.