There is no obvious boundary information in Chinese reading. It has been shown that the introduction of word boundary information presented with alternating colors without changing the text distribution could significantly improve the reading speed of Chinese children in grade 2 (Perea and Wang in Mem Cognit 45(7):1160-1170, 2017. https://bibliotheek.ehb.be:2102/10.3758/s13421-017-0717-0). However, few studies have examined how the effect of word boundary information on children's oral reading develops and changes as children's grade increases. The present study asked Chinese children in grades 2-5 to read alternating-color and mono-color text orally and used eye-tracking technology to explore the developmental trajectory of the influence of word boundary information on oral reading. The results indicated that children in grade 2 and grade 3 showed faster reading speeds in the alternating-color condition than in the mono-color condition. In contrast, there was no difference between the two conditions in children in grade 4 and grade 5. We discuss the mechanisms of the findings and the implications for education.