To avoid the washout loss of solidified silt in underwater applications, the main factors that affect the washout resistance of solidified silt are analyzed and the grout viscosity is adopted to quantify the washout resistance based on underwater tests. Test results indicate that the viscosity is the intrinsic factor and the grouting height is the extrinsic factor that affect the washout resistance of solidified silt. The greater the viscosity and the smaller the grouting height, the greater the washout resistance of the solidified silt. HPMC is more suitable for underwater solidified silt than GG and PAM due to its high thickening effectiveness, and the pH and transmissivity methods can be used to characterize its performance on washout resistance. Both falling and collisional diffusions will occur when the viscosity is smaller than 3.7 Pa·s, only collisional diffusion will happen when the viscosity is varying from 3.7 to 22.0 Pa·s, but no diffusion is spotted when the viscosity is greater than 22.0 Pa·s. Solidified silt with viscosity varying from 3.7 to 22 Pa·s and grouting height smaller than 30 cm is recommended for underwater applications.