This paper evaluates the multistage adaptive test (MST) design of a large-scale academic language assessment (ACCESS) for Grades 1-12, with an aim to simplify the current MST design, using both operational and simulated test data. Study 1 explored the operational population data (1,456,287 test-takers) of the listening and reading tests of MST ACCESS in the 2018-2019 school year to evaluate the MST design in terms of measurement efficiency and precision. Study 2 is a simulation study conducted to find an optimal MST design with manipulation on the number of items per stage and panel structure. The results from operational test data showed that the test length for both the listening and reading tests could be shortened to six folders (i.e., 18 items), with final ability estimates and reliability coefficients comparable to those of the current test, with slight differences. The simulation study showed that all six proposed MST designs yielded slightly better measurement accuracy and efficiency than the current design, among which the 1-3-3 MST design with more items at earlier stages ranked first. The findings of this study provide implications for the evaluation of MST designs and ways to optimize MST designs in language assessment.