The purpose of this study is to observe and analyze the Beijing dialect from the perspective of the lexicon. It sought to determine the lexical differences between the Beijing dialect and Putonghua. In the first part, it introduces the connection between the Beijing dialect and Putonghua and describes in details their individual developments in different historical stages. These imprints of the evolution of language in history have made distinctions between the Beijing dialect and Putonghua. The second part of this paper analyzes the lexical features of the Beijing dialect based on four aspects: the characteristic of word-formation, variant pronunciations of mono-syllabic words, usage of Measure words, and loanword from the Mongolian and Manchu language. First, in the basic Chinese vocabulary, there are about 47.11% of words that are different between the Beijing dialect and Putonghua. The differences between these words are mainly due to differences in partial morphemes, differences in the number of syllables, and the presence or absence of retroflex ending. Second, for the pronunciation of mono-syllabic words, the differences mainly occurs in tone sandhi, changes in finals or initials and dissimilation of the phoneme. Third, the characteristics of Measure words in the Beijing dialect use nouns indicating body parts. Fourth, most loanwords from the Mongolian or Manchu language are written in the way of phonetic loan characters. Some of the phonetic loan characters are associated with the meaning of the borrowing words, but some are not. The third part of this paper expounds on the direction of development of the Beijing dialect and Mandarin from the aspects of urban population structure; the people's attitudes toward language and the language environment in the Beijing dialect and Putonghua; and government policy intervention. In general, the vocabulary of the Beijing dialect is gradually losing its characteristics and becoming more and more like Putonghua.