The soil rectangular blanket, whose design method has been already built up, is widely used as measures against the seepage from a reservoir of embankment dam or an irrigation tank on pervious foundation. This paper demonstrates an analytical solution of a seepage model for a triangular blanket and reveals that the analytical solution for the triangular blanket and the resultant solution for the seepage discharge can be successfully expressed by introducing the modified Bessel function of the first kind of orders 0 and 1. Compared with the seepage discharge of the model and the numerical solution by Boundary Element Method (BEM), the seepage discharge is shown to be accurate when the blanket length is practicable, though it is overestimated when the blanket length is relatively small. Moreover applying both the analytical solution of the model and the BEM solution to a large number of model cases, in each of which the length and the area (volume) of the triangular blanket are the same as those of the rectangular one, it is concluded that the triangular blanket is more effective against the seepage.