Since independence，the Tanzanian government has attached great importance to the standardization and promotion of the Swahili language as one of the key instruments in building national cohesion. This policy has successfully made Swahili the means of communication in Tanzania. However，despite the state’s continued emphasis on language standardization，urban Tanzanians have created and developed a more colloquial form of expression，namely the Kiswahili cha Mtaani（Street Swahili）. As early as the 1950s，Kiswahili cha Mtaani（Street Swahili）attracted the attention of scholars. In the late 1980s，it developed rapidly in urban areas，and most of its users were urban youth. This article collates the data of the author’s field survey in Tanzania from 2008 to 2016. By comparing the previous representative Street Swahili dictionary，this article summarizes the evolution and characteristics of Kiswahili cha Mtaani（Street Swahili）with a large number of examples，analyzes how its creators practiced discourse in the context of modern society，and discusses the relationship between this discourse practice and modern social change in Tanzania. We found that Kiswahili cha Mtaani（Street Swahili）embodies social and cultural attributes，as well as its contemporary significance. Its rapid growth was amplified amidst the waves of political liberalization and economic reform within the country. While the importance of discourse practice of the youth established a bastion within the Swahili Studies in the new millennium，most of the Kiswahili cha Mtaani（Street Swahili）vocabulary remain decipherable thanks to the mutual inclusiveness of Street Swahili and its standard langue. This unique inclusiveness，together with the vital usage of Kiswahili cha Mtaani in different places，are all embedded into the changes of modern society.