Abstract: According to the principle of similitude, a simulation experimental device for investigating the motion states of bottom drilling string in a horizontal well was designed and set up. Some experiments have been done by means of the device under different weights on bit and rotary speeds. The experiments result shows that the bottom drilling string exhibits three kinds of motion states at different rotary speeds, including approximate circular swaying, a form of “8” swaying and a flat form of “8” swaying. With the increase of the rotary speed, the motion states of drilling string will be transformed from approximate circular swaying into a form of “8” swaying and a flat form “8” swaying. The weight on bit and rotary speed are the main factors affecting the motion behavior of the bottom drilling string in a horizontal well. The higher the rotary speed is, the more easily the form of “8” swaying will be formed. Increasing the weight on bit is advantageous to form “8” swaying appearance at the bottom drilling string. The motion range of bottom drilling string at a horizontal well is always in the lower right of the wellbore.