Substances with superparamagnetism, a special magnetic effect, are magnetized under external magnetic field. When the applied magnetic field disappears, superparamagnetic substances have almost zero magnetization intensity, with quite small coercivity. The performance of magnetic powder or magnetic suspension used in magnetic particle detection greatly affects detection sensitivity. This paper studies a new method for preparing nano-superparamagnetic non-destructive testing materials by combing membrane technology and chemical co-precipitation method. Magnetic nano-ferroferric oxide particles were prepared by preparing membrane reactor based on chemical co-precipitation method to find the optimal synthesis conditions for nano-ferroferric oxide particles. Polymer nano-magnetic materials with good dispersibility, superparamagnetism, high dispersibility, uniform particle size distribution were obtained, which can well detect cracks, especially small cracks. It is a good magnetic nondestructive testing material.