In this paper,the freezing damage roadbed at K136+600~K136+938 of Da Zhun was taken as an engineering example,which was treated by the dewatering drainhole. With the aid of the finite element software, the drainage effect which could be affected by the spacing, aperture, angle, length of the dewatering drainhole were studied by numerical simulation methods. Result indicated that: (1)The spacing should be selected by size of subgrade, the spacing can be taken as 1.0m.(2)When the aperture is taken as 0.1m~0.2m, the drainage effect is the best.(3)The angle can be usually taken as 3°~6°.(4)The length should cross the roadbed in the transverse direction, the drainhole should be staggered on both sides of the roadbed, and the length can be taken as the half of the subgrade width.(5)For this railway, the drainage hole should be set 1.5m below the subgrade surface. The research results provide theoretical guidance for the design of dewatering drainhole in existing railway subgrade freezing damage.