Usually, there are some “light rings” effect which like shadows and close to the ground in 3D games. This paper analyzes the relationship between the “Light Ring” effect and the terrain in 3D game, and hands out an practical algorithm of the effect mentioned above. With this algorithm, the “Light Rings” will follow characters, close to the ground, as the ups and downs of the ground. The algorithm can combine with a variety of game engine, and has been applied successfully in a lot of 3D games.