The present study investigated the development and influence of working memory abilities (WM) and number sense (NS) on mathematics achievement in junior high school students (grades 7–9, N = 267). Math achievement was measured by three sectional examinations in a semester, NS was indicated by an approximate numerosity system task, and WM was assessed by a battery of four tasks. Developmental trends in both WM and NS task scores were observed. Memory updating (MU) in the WM tasks was found to be dominant in predicting math achievement in correlation and regression analyses. A similar pattern was observed for separate analyses across grade levels, except that in grade 7 a significant unique contribution of NS to math was observed after taking MU into account. The findings suggest that WM ability (especially that used in MU task) had greater influence on math achievement than NS.