Abstract: In 1979, Fu and his colleagues presented their work of realizing Maxwall's demon, theoretically and experimentally, in Journal of Shanghai Jiaotong University. Fu declared that a permanent current was obtained in their device. This means that a new machine of the seeond kind of perpetual motion have been invented.In this paper, the author gives a detail anslysis of Fu's work, and the following conclusions have been obtained: (1) In Fu's experiment, the object controlled by the magnetic field was the mass-motion of electrons, not the individual electron. Magnetic field did not separate electrons individually into high-speed group and low-speed group, for this reason, it is not a demon. The process which occured in the whole experiment was a mass-flowing process of numerous amount of particules. this must be belong to the area of thermodynamics. (2) The current flowing in Fu's device is actually the initial velocity current of a vacuum diode. (3) The initial velocity current is in turn a changed form of Thomson thermoelectric effect, which necessitates a hot tank and a cold tank to work, obeying the second law of thermodynamics. (4) The role of action of the magnetic field is to control the mass-motion of electrons, causing a weak rectification effect in the device. The current should vanish if there is no this effect, as the initial-velocity elelctrons should eliminates each other.