Responsive polymer gels, which can undergo sol-gel/gel-sol transition by responding to external physical or chemical stimuli, have attracted significant attention over the past decades. However, implementing the sol-gel-sol transition under constant temperature remains a major challenge. Herein, a novel polymer gel (HGX), which can undergo a sequential sol-gel-sol transition upon simply heating at a fixed temperature, was developed based on poly (2-hydroxyethyl methacrylate) (PHEMA) and glyoxal (GX). The gelation and degradation times could be tuned over wide ranges (10-1500 min and 1.5 h ~ 15 days, respectively) by changing GX content and temperature. After gelation, the elastic modulus of HGX was as large as 847 Pa. Before gelation or after degradation, the apparent viscosity of HGX was below 30 mPa·s. This unique transition is mainly ascribed to the dynamic competition between aldolization and hydrolysis of ester.