Based on the data of the research articles on System (a representative journal of applied linguistics) during 24 years, the study focuses on the diachronic development of six types of words, i.e., general adjectives, general adverbs, emotional adjectives, emotional adverbs, positive adjectives, and positive adverbs, exploring the correlation between the six types of vocabulary and text readability with the natural language processing research method of POS tagging and sentiment analysis. The findings are as follows: 1) Linguistic positivity bias is verified in this study; 2) there is a negative correlation between adjectives, adverbs and text readability; 3) emotional adjectives and positive adjectives fail to significantly predict the diachronic development of adjectives, while emotional adverbs and positive adverbs can significantly predict the diachronic development of adverbs. The study is of reference value for improving the readability of academic papers and academic writing and publishing.