This paper developed academic vocabulary test to quantitatively evaluate learners’ academic vocabulary ability, and qualitatively analyzed learners’ reading ability. First, textbook for 3rd grade middle school were made into a corpus, and the ratio of basic vocabulary and academic vocabulary. As a result, basic vocabulary accounts for only 6.54% of the total individual vocabulary, but covered 41% of the textbook. Academic vocabulary only account for 7.3% of the total individual vocabulary, but covered 20% of the textbook. As a result of the academic vocabulary test, the average score of learners was 16.97 points, and the average score did not reach 70% of the perfect score. Academic vocabulary showed a 20% coverage of textbooks. As a result of interpreting the learners’ academic vocabulary ability as Threshold Hypothesis, the academic vocabulary ability was the core ability to decide whether to read textbooks. Learners whose academic vocabulary ability are lower can not read the textbook by paying attention to academic vocabulary, failing to understand the details of the textbook.