The permeability of the tight reservoir was low，and there were many natural and artificial cracks，resulting in serious CO2 Channeling. the injection performance and blocking ability of the modified starch gel（MSG）system was evaluated by measurements of the viscosity and the gel strength under different concentration. the sealing effect of MSG system on the fractured tight sandstone core with 3 different fracture opening degrees was studied，and the effect of improving the recovery rate after plugging was studied. Furtherly，the MSG plugging limits of CO2 flooding in fractured tight sandstone reservoirs was probed. The experimental results showed that the viscosity of MSG system was low before gelling，which was beneficial to the smooth injection of the system，and the strength was high after the gel formation，indicating that MSG system could be used for the sealing of crack in experimental condition. Under the condition of 0.42 mm crack opening，the blocking rate of MSG system was more than 99%，the breakthrough pressure was up to 24.9 MPa and the enhanced oil recovery was 28% after plugging. Under the condition of 0.65 mm crack opening，the blocking rate was 92%，the breakthrough pressure was reduced to 15.9 MPa，and the enhanced oil recovery rate after plugging was 18%. Under the condition of 0.08 mm crack opening，the injection performance of MSG system was poor，the blocking rate was 90%，the breakthrough pressure was 3.6 MPa，and the enhanced oil recovery rate was 9.8% after plugging.The MSG system was adaptable to the fractured tight sandstone reservoirs with about 0.42 mm crack opening