During development of NH-A oilfield by gas injection，the gas-oil ratio measured in oil wells increases sharply， indicating the obvious gas channeling. To address this issue，a seawater-based and high temperature gel system was developed. Non-ionic polyacrylamide（NPAM）and phenolic system were selected as the main agent and crosslinking agent，three methods were explored to enhance the high-temperature stability of the gel by increasing polymer concentration，adding deaerators to inhibit polymer degradation and adding high temperature stabilizer. The crosslinking agent，high temperature stabilizer and polymer concentration optimization were carried out to form a seawater-based high temperature gel system. Finally，the natural gas tolerance and plugging ability of the system were evaluated. The results showed that，although the high-concentration polymer could enhance the thermal stability of gels，the dehydration rate of single-high-concentration（0.8%）non-ionic polyacrylamide gel was more than 90% after ageing for only 48 h. The presence of deoxidizer was also difficult to significantly enhance the thermal stability of gel. Fortunately，the high-temperature stabilizer could inhibit the interaction between the calcium，magnesium ions and the polymer， thus，obviously enhanced its temperature resistance，which was the key to the construction of seawater based high temperature gel system. The final gel formula formed by component-concentration optimization was：0.4% NPAM + 0.3% urotropine + 0.3% resorcinol + 0.8% stabilizer. It could maintain excellent colloidal state under high calcium，magnesium ion concentration，high temperature of 125 ℃ and natural gas condition，with a elastic modulus greater than 1500 mPa and a dehydration rate less than 10%. The gel could coexist with natural gas under high temperature and high pressure environment without breaking. It could form a good plugging on the core with a residual resistance coefficient of 210，a plugging rate of 99.5%，and a high scouring resistance.