The current study characterized the spatial selectivity of numerosity adaptation. In Experiment 1, adaptors were arranged vertically with 8 dots at the top of the visual field and 400 dots at the bottom, and participants' perceived magnitude in the left field decreased compared to that in the right, as revealed in the numerosity comparing task after adaptation. In contrast, the perceived magnitude in the right field decreased compared to that in the left with inversed adaptors (400 dots at top, 8 at bottom). In Experiment 2, adaptors were presented horizontally, and they showed no significant effect on numerosity perception, which was tested vertically. This study demonstrated that numerosity adaptation along the vertical orientation could affect numerosity perception along the horizontal orientation, and the latter was affected by the former according to a rule of associating "top" with "right" and "bottom" with "left." The spatial selectivity of numerosity adaptation showed distinguishing features that should function to abstract spatial relationships rather than create purely retinotopic mapping. We proposed that numerosity adaptation is based on spatial-numerical-associated codes. Vertical adaptors could activate both the vertical and horizontal Mental Number Lines (MNLs) and involve an interaction between these types of MNLs. According to behavioral data, horizontal adaptors showed no significant influence on perception along the vertical orientation, which might be due to the higher threshold required to activate the vertical MNL.