This study examined the development of text complexity for the past 25 years of reading comprehension passages in the National Matriculation English Test (NMET) in China. Text complexity of 206 reading passages at lexical, syntactic, and discourse levels has been measured longitudinally and compared across the years. The natural language processing tools used in the study included TAALES, TAALED, TAASSC, and TAACO. To compare the differences across the years at various levels of text complexity, ANOVA and MANOVA tests were conducted. The results suggested that lexical level text complexity revealed the most evident changes throughout the years, lexical sophistication, density, and diversity levels of the most recent years of reading passages have increased remarkably compared to the early years. The syntactic level text complexity indicated a moderate elevation toward the recent years of reading passages. For the discourse level text complexity, regarding cohesion, insignificant fluctuation occurred throughout the years and the general trend was not necessarily increasing. Combined, the results indicated that text complexity of the reading comprehension passages in the NMET over the past 25 years had been steadily increasing by including more low frequency and academic vocabulary, diversifying vocabulary in the passages, and complicating sentence and grammatical structures. The results were further examined against the general curriculum standards and guidelines to analyze whether the changes were reflected in the policies. It showed that the exams required a much larger vocabulary size than the number indicated in the guidelines, suggestions for test designers and pedagogical practices were provided accordingly.