Mixed soil exists in hydraulic reclamation project. Regarding the problem of large settlement of sludge in foundation treatment，this paper studies the relationship between the settlement velocity，flow diameter and outlet of hydraulic reclamation soil particles with critical particle size，The theoretical settlement velocity of critical particle value is obtained by using Stokes law. The width of the outlet for discharging critical soil particles is set by combining the flow diameter，outlet velocity and particle settlement velocity. Taking the thin-walled weir as an example，other weirs can be calculated by the corresponding flow formula. Through the analysis and calculation of critical conditions，the contradiction between hydraulic reclamation construction and foundation treatment is solved，and reasonable thin-walled weir parameters for loss control are sought to provide reference for practical engineering applications.