Eye-movement technology has been often used to examine reading processes, but research has seldom examined the relationship between the reading process and comprehension performance, and whether the relationships are similar or different across grades. To investigate this, we conducted a 3-year longitudinal study starting at grade 4, with 175 effect samples to track the development data of eye movements on text-and-diagram reading. A series of temporal and spatial eye-movement predictors were identified to predict reading comprehension in various grades. The result of a hierarchical regression model established that total fixation duration measures (reflects processing level) and mean fixation duration (reflects decoding efficiency) were relatively better predictors of the post-reading tests at grades 5 and 6. That is, the readers made more mental efforts and had better decoding ability, which predict better post-reading test scores. Meanwhile, in grades 5 and 6, rereading total fixation duration on both the main text and diagrams consistently predicted the post-reading tests, indicating that the readers' self-regulated study time on re-processing some specific information is important for reading comprehension. Besides, a longitudinal structural equation modeling (SEM) revealed that the readers' fixation durations and text-and-diagram regression count in the lower fourth grade could significantly predict the same indicators in the following 2 years. In summary, this study identified the critical eye-movement indicators for predicting reading-test performance, and these predictions were more effective for the readers in upper grades than for those in the lower grades.