The invention relates to a handheld ultralow field MRI (magnetic resonance imaging) system based on an SQUID (superconducting quantum interference device). The system provided by the invention comprises a hardware part and a software part, wherein the hardware part encapsulates a polarizing coil module, a gradient coil module, a receiving coil module, a cooling module and the SQUID in a detectionhead housing; and a magnetic field parameter transformation module is arranged in the software part. After the polarizing coil module, the gradient coil module, the receiving coil module, the coolingmodule and the SQUID are encapsulated in the detection head housing, the cooling module provides an ultralow temperature working environment for the SQUID without using liquid helium cooling; and themagnetic field parameter transformation module transforms irregular magnetic filed parameters detected by the SQUID into the magnetic field parameters suitable for using the existing MRI algorithm, so that the existing MRI algorithm can be used for reconstructing images to achieve the handheld of the MRI system. The system has low operation and maintenance costs, can substitute for 0.1-1 T permanent magnet MRI equipment and is suitable for medium and small-sized hospitals.