This study aimed to clarify whether the down regulation of K+-Cl−co-transporter 2 (KCC2) in the sacral parasympathetic nucleus (SPN) of the lumbosacral spinal cord, from which the efferent pathway innervating the bladder originates, causes cellular hyperexcitability and triggers detrusor overactivity (DO) in spinal cord injury (SCI). SCI was produced by the Th8-9 spinal cord transection in female C57BL/6 mice. At 4 weeks after SCI, CLP290, a KCC2 activator, was administered, and cystometry was performed. Thereafter, neuronal activity with c-fos staining and KCC2 expression in the cholinergic preganglionic parasympathetic neurons in the SPN were examined using immunohistochemistry. Firing properties of neurons in the SPN region were evaluated by extracellular recordings in spinal cord slice preparations. DO evident as non-voiding contractions (NVC) was significantly reduced by CLP290 treatment in SCI mice. The number of c-fos-positive cells and co-expression of c-fos in choline acetyltransferase (ChAT)-positive cells were decreased in the SPN region of SCI-CLP290 group vs. SCI-vehicle group. KCC2 immunoreactivity was present on the cell membrane of SPN neurons, and the normalized fluorescence intensity of KCC2 in ChAT-positive SPN neurons was decreased in SCI-vehicle group, vs. SI-vehicle group, but recovered in SCI-CLP290 group. Extracellular recordings showed that CLP290 suppressed the high frequency firing activity of SPN neurons in SCI mice. These results indicated that SCI-induced DO is associated with downregulation of KCC2 in preganglionic parasympathetic neurons, and that activation of KCC2 transporters can reduce DO, increase KCC2 expression in preganglionic parasympathetic neurons and decrease neuronal firing of SPN neurons in SCI mice.