An in situ terpolymer gel system formed by resorcinol hexamethylenetetramine (HMTA) was developed and systematically evaluated for water management in extremely high-temperature reservoirs. Suitable gelation time and favorable gelation performance were obtained by adjusting terpolymer and/or cross-linker system concentrations. With the increase of the terpolymer, the resorcinol HMTA concentration increased both the gelation time and gelation performance. The gel system was prepared by deionized water and maintained good thermostability in a high-salinity environment. Very low concentrations of NaCl, KCl, and CaCl2 can delay gelation time. After the critical concentrations are reached, these inorganic ions will boost the cross-linking reaction; however, the presence of MgCl2 shortens gelation time. The gel system in ampules was kept stable at 150 degrees C for 5 months, and the differential scanning calorimetry measurement indicated that the gel system could be used for water management at temperatures up to 240 degrees C. Uniformly distributed three-dimensional network microstructures and dendritic structures among the gel grid pores could further increase the network structure strength and firmly lock water within the gel even under extremely high temperatures. The use of the gelation mechanism of an in situ terpolymer gel system formed by resorcinol HMTA can help petroleum engineers control gelation time and performance.