The utility model discloses a window sill for a building outer window and belongs to building components of industrial and civil construction buildings. Due to water seepage of an existing building outer window sill, the service life of a heat preservation system is shortened, and a sill lower portion wall body is polluted. According to the window sill, an oblique base face, a locating board, a lower rain blocking board and two side waterproof boards are formed in a board in a bending mode, the locating board is formed by the upper edge of the board in a bending mode, the locating board is a plane and is connected with the base face through a vertical wall, the lower rain blocking board is formed by the lower edge of the board in a bending mode and extends downwards from the lower end of the base face, and the two side waterproof boards are formed by the two side edges of the board in a bending mode and extend upwards from the side of the base face. Rainwater on the outer side of the window and on the sill can only flow to the ground along the window sill, the rainwater cannot flow to the sill and the two sides of the sill in a leakage mode, the wall body on the lower portion of the sill is protected, the rainwater cannot pollute the wall body, and a building can be clean and attractive. The structure is simple, batch manufacturing can be easily carried out, and mounting cost is low.