SUMOylation is an essential and highly dynamic post-translational modification that regulates developmental processes and stress adaptations in plants to environmental cues. The global SUMOylation is quickly induced by dehydration and hyperosmotic stresses in plants, while the detailed mechanism underlying such SUMOylation dynamics is largely unknown. Herein, we report key components in osmotic stress and abscisic acid signaling, SNF1-related protein kinase 2s (SnRK2s), phosphorylate SUMO E3 ligase SAP and MIZ1-1 (SIZ1) to stabilize SIZ1 protein under stress conditions. The Ser820 residue is a functional SnRK2 phosphosite and non-phosphorylatable SIZ1S820A is unstable in vivo and in vitro. Under osmotic stress, the rapidly activated protein kinases SnRK2.4 and SnRK2.6 can phosphorylate SIZ1, which enhances the stability of SIZ1, and participates in the response to osmotic stress. However, the instability of SIZ1S820A protein was independent of COP1, indicating that there might be a new degradation mechanism for SIZ1. Our studies suggest that SnRK2s induced phosphorylation regulates SIZ1 stability to precisely modulates SUMOylation dynamics upon osmotic stresses. Additionally, such a reciprocal regulation by SnRK2 and COP1 might be a general regulation machinery that controls the stability of dozens of proteins.