In view of the poor real-time performance and the lack of diverse monitoring means of the greenhouse monitoring system, a cloud-based smart greenhouse monitoring system is designed using wireless sensor networks technology and the cloud technology. The system takes the cloud server as the core, and users can access the cloud server remotely through the PC terminal or mobile phone online, so as to realize the real-time monitoring and control of the greenhouse environmental factors(temperature, moisture, illumination). In addition, in order to avoid atmospheric pollution caused by excessive greenhouse gas emissions, the system adds on-line monitoring of the main greenhouse gases (CO2, CH4) in the greenhouse. With this system, we can achieve the goal of increasing crop yield and reducing environmental pollution. The experimental results show that the system has high stability and accuracy, and is easy to operate. It is suitable for popularization and application in greenhouses.