In order to solve the problems of high injection pressure and difficult injection in the injection process of conventional polymer gel，the hydrogel system with low initial viscosity and easy degradation was prepared，and the injection and gelation properties of the gel system were investigated. The microstructure and mechanical strength of the composites were studied by scanning electron microscopy（SEM）and universal mechanical testing machine. Furthermore，the heat resistance and thermal stability of the gel were evaluated by thermogravimetric analysis and gel dehydration rate test. Finally，the degradability and plugging performance of the gel were evaluated by degradability experiments and physical simulation experiments. The results showed that the formulation of seawater-based hydrogel system suitable for unconsolidated sandstone reservoirs with low initial viscosity and easy degradation was as follows，0.2 % polymer VC-1+2% itaconic acid+0.7% melamine+0.1% sodium thiosulfate， and the gelation temperature range of the gel system was 50— 90 ℃，the gelation time was regulated within 5—22 h，and the compressive strength（90 ℃）was 310.4 kPa，exhibiting high mechanical strength. The dehydration rate of the gel at 90 ℃ for 90 d was only 7.4%，and the maximum temperature resistance reached 201.1 ℃，showing good long-term stability and thermal stability. The gel had good plugging and degradability. In the physical simulation experiment，the breakthrough pressure of the gel reached 3.99 MPa，and after the first water flooding breakthrough，the secondary migration plugging could be carried out. When the mass ratio of the degradation solution QC-1 to the gel was 1∶2，the degradation rate of the gel reached 98.2% after aging for 10 days at the temperature of 70 ℃. When the water plugging operation was carried out by using the developed hydrogel system，the average daily oil production of typical wells increased from 2.3 t to 62 t，the water cut decreased from 99.7% to 72.4%，and the crude oil production increased by stages reached 2710 t，which could significantly increase oil well production and reduce water cut.