With the accerleration of globalisation, the importance of English as an international common language has become more and more prominent. This study investigated the negative transfer effect of Shanghai dialect on English vowel acquisition in Shanghai high school students. Through the analysis of formant data of English vowel pronunciation in 49 Shanghai high school students, it is found that there is no significant change in tongue position when students pronounce some long and short vowels, the overall tongue position is low, and the distinction between long and short vowels is not obvious. The results show that the specific vowel phonemes in Shanghai dialect have a negative transfer effect on English pronunciation, which affects the accuracy of students' English pronunciation. Based on this, this study puts forward a variety of methods to improve English pronunciation teaching, including strengthening tongue position and oral shape training, using multimedia technology to assist teaching and providing personalized tutoring to help students overcome pronunciation barriers caused by dialects.