Gradient magnetic field driving method does not have strict structural requirements for micro robots, making processing convenient. However, the speed control parameter of micro robots under gradient magnetic fields is single, which limits the practical application of this type of robots. Therefore, this article proposes to use alternating signals to control the motion speed of magnetic microsphere robots. Here, a motion analysis model of a magnetic microsphere robot driven by alternating signals is established, and a formula for calculating the movement speed of the magnetic microsphere robot is provided. The special motion mode of the magnetic microsphere robot driven by alternating signals is considered, and the correctness of the theoretical calculation is verified by experiments. The mechanisms and conditions for the different motion modes of magnetic microsphere robots are revealed. The movement speed of the magnetic microsphere robot under the alternating voltage signals is determined as a function of signal frequency, signal duty cycle, and bias voltage value. The results show that under the alternating voltage signals, the magnetic microsphere robot has three different motion states, namely rotation -movement alternation, swing-movement alternation, and pure movement. There are three control parameters of the speed, which are signal change frequency, duty cycle, and bias voltage.