A spiral-type biotic tract robot moving through biotic tracts and its outer system producing an invariable magnetic field strength at a steady rotational speed are proposed in the paper. The robot has a spiral surface and two strong magnet bars under the spiral. The outer rotational magnetic field is produced by the circumferential array of permanent magnets. During working, the robot locates in the center point of the array all along because the magnetic field in the center point possesses an invariable rotating speed and magnetic strength so that the magnet field applies an invariable torque on the robot to make it revolve around its axis. During revolving, the counterforce of intestine on the spiral surface of the robot pushes it forward or backward. Therefore, the autonomous motion of the robot is realized. The circumferential array method of permanent magnets is given and magnetic inductive intensity distribution on a sphere surrounding one permanent magnet is calculated. Then, the computing formulas of the magnetic field intensity in the center point of permanent magnets array are deduced by the method of coordinate transformation. It is found that an invariable rotating speed and strength of the rotating magnetic field is achieved if the number of permanent magnets is no less than three. Analysis shows that the magnetic field strength in the center point is proportional to the number of permanent magnets, so it is easy to get specified magnetic field intensity. Experiments are carried out in a segment of small intestine of pig. The experiments show that the robot works reliably which can run forward and backward at controllable and changeable velocity when the diameter of the samples is less than 20 mm. It is found that when the intensity of the outer magnetic field is about 500 Oe, the active torque on the samples is about 1.0 mN.m. At present, the tract robot has been used be a carrier, such as a temperature sensor or a wireless capsule endoscope.