At present, green plants exist in all aspects of our lives. And statistics shows that the pest species of green plants is very large. Thus accurate and rapid diagnosis is regarded as an essential component of green plant protection. Moreover, we couldn’t find relevant information easily. That is why pest diagnosis is difficult and inefficient for technicians and farmers. In view of the above, the expert systems have been widely used in pest identification. However, most of traditional expert systems for assistant diagnosis of green plant pests are based on dichotomous structure. They are not flexible enough and only equal the electronic dichotomous keys. Compared with dichotomous structure, the system based on multi-branch structure has more advantages for accurate and rapid diagnosis. This paper describes the design and development of a web-based green plant pest expert system as part of Tianjin science and technology cooperation project. Based on user needs, Tianjin Planting Pest Assistant Diagnosis System (TPPADS) was developed with ASP.NET, C# and Microsoft SQL server 2008 database. It can show many features simultaneously. Meanwhile, data maintaining is also very easy and simple as same as the Microsoft Windows Explorer. The system included about more than 300 species of green plant pests. Diagnosis knowledge was obtained from Tianjin Institute of Plant Protection. TPPADS can be used as a diagnosis tool and information database both for plant protection professionals and farmers. We believe its application prospect should be well.