Abstract: For many space remote sensors, the earth is an important extended source of stray light. An algorithm suited for use on PC computer to calculate this kind of stray light is described. The earth surface is substituted by plane area according to the radiance conservation theorem. Then the plane arm is divided into elements, and they are treated as point sources with different incident angles. The earth's contribution is the sum of those of the plane elements. The prediCtions of a typical systems on two typical orbital altitudes show that, the stray light due to the earth scattering for low orbital sensors should be carefully deal with.