Relying on the ship lock cofferdam with a large head difference in Wan’an，Jiangxi Province，this study adjusts the water-binder ratio，the bentonite content，and other parameters of the plastic concrete mix used in the cutoff wall according to water resources and hydropower engineering standards. The paper determines the optimal plastic concrete mix ratio（the water-binder ratio is 0.80，and the bentonite content is 25%）with high ultimate strain，low compressive strength，a low elastic modulus，and a low permeability coefficient for cofferdam with a large head difference，then tests the plastic concrete by the field water injection test and the high-density electrical method. The results show that the plastic concrete cutoff wall has excellent impermeability，good section continuity，and good overall integrity. During the service period of the cofferdam，the strain monitoring results in the plastic concrete cutoff wall indicate that the plastic concrete is in a safe compressive state，and the cutoff effect of the wall is good.