SQUID is a kind of flux sensor which converts flux into voltage. The basic principle of SQUID is based on the superconducting Josephson effect and the polarization of flux. Based on SQUID, various sensors and measuring instruments are derived, which can be used to measure physical quantities such as magnetic field, voltage, magnetic susceptibility and so on. Two superconductors separated by a thin barrier layer form a Josephson tunnel junction. When the superconductor closed loop containing Josephson tunnel junction is biased by the appropriate size of the current, there will be a macroscopic quantum interference phenomenon, that is, the voltage at both ends of the tunnel junction is a periodic function of the external magnetic flux change in the loop hole of the closed loop, the period is a single magnetic flux of 0=2.07×10-15Wb, such a loop is called the superconducting quantum interferometer.