Metal substance detection plays an extremely important role in daily life, industrial manufacturing and even industrial security. The traditional methods include optical detection, X-ray detection, microwave detection and ultrasonic detection. These methods, playing a vital role in the field of non-destructive testing, can not only judge the presence or absence of metal, but also accurately detect the type and size of metal defects. For mi-crowave detection, the detection efficiency of metal materials is limited by the response sensitivity of the de-tector to microwaves. In recent years, scientists have discovered a quantum sensing system based on the dia-mond nitrogen-vacancy (NV) color center. The system obtains optical detection magnetic resonance (ODMR) fluorescence spectra under the combined action of a 532nm laser and a certain frequency band of microwaves, and the signal contrast changes significantly with the microwave power. Based on the NV color center quantum sensing system, this paper studies its application in the field of metal detection, and takes steel detection as an example to detect the size of steel bars according to the changes in the spectral line, providing a new method for non-destructive testing such as metal substance detection.