Abstract: Since OMG adopted UML as its standard modeling language in 1997, UML has got universal attention in computer science and engineering, and found wide application It has become a de facto standard But at the same time, UML suffers criticism from users, vendors, scholars, and teachers with different point of views OMG has issued three revisions （1 3, 1 4 and 2 0） of UML, and different people put up different opinions on UML with emphasis on simplifying, defining kernel, extending mechanism and formal semantics, etc