System for environmental monitoring and automatic irrigation in the growing field of radix ophiopogonis based on wireless sensor network technology is designed and implemented. The system features the design of hardware modularity which separates the power supply unit from the sensor nodes, the in-field wireless sensor network to monitor the diversifying field environmental data, and the integrating decision-making model for the irrigation control of the field of radix ophiopogonis. In the system, the collected field data, such as the humidity and temperature of the soil, combined with the weather data including the air temperature, humidity, and rainfall from meteorological observatory, are served as the input of the decision-making model to make the decision of irrigation. The decisions actuate the feedback loops to reach the goal of field monitoring and automatic irrigation control. The operation results show that the functions of our system running in the field have high stability and reliability, and the system can be suitably applied to the large area of farmland of medicinal herbs.