Previous studies have uncovered the relationship between handwriting and mathematics performance, without articulating the underlying cognitive mechanism. With 197 Chinese fourth graders as participants, this study explored the mechanism by which the spacing/spatial relationship in handwriting legibility affects number-line estimation through VMI. The results indicate that (1) the spacing/spatial relationship in handwriting legibility had a direct and positive effect on students' number-line estimation performance; (2) VMI completely mediated the relationship between the spacing/spatial relationship in handwriting legibility and number-line estimation. Overall, the findings confirmed one specific mechanism of linguistic influences on students' numerical knowledge. Possible explanations for the results and implications for future research are discussed.