Acrylamide copolymer hydrogels used in profile modification applications in an oilfield share a common problem, termed syneresis, which decreases the efficiency of profile modification. In this paper, sodium tripolyphosphate (STPP) was found to be an effective syneresis inhibitor for the hydrogel formulated with acrylamide: copolymer of acryloyloxyethyl trimethylammonium chloride (AM/DAC), and an ultrastable hydrogel for enhanced oil recovery in high-temperature and, high-salinity petroleum reservoirs was obtained on the basis of double-groups cross-linking. Experimental investigations, including DLS, FTIR, NMR, SEM, and core flood test, have been conducted to elucidate the mechanism of STPP inhibiting the hydrogel syneresis in the aspect of the reaction between STPP and AM/DAC. The result showed that the AM/DAC cross-links with STPP on the basis of the hydrolysis reaction of the ester group and STPP, Whereby the new bond of C-O-P is formed. For this reason, the viscosity and hydrophilicity of AM/DAC were signifiCantly increased by STPP, aria the generated donble-groups cross-linking (AM/DAC cross-linked with phenol-formaldehyde, AM/DAC cross-linked with STPP) made more AM/DAC molecule chains cross linked together therefore, the stronger grid structure was formed. Therefore, the increase of the hydrophilicity and the improved stability of grid structure enhanced the water holding capacity of the hydrogel, leading to the decrease of the hydrogel syneresis and the increase Of the Water-shutoff efficiency.