The invention discloses a nuclear magnetic resonance imaging observing device and method in a coal rock crack development process. The device comprises a nuclear magnetic resonance imaging analyzer and a rock core vacuum saturation instrument; the method provides a low-field magnetic resonance technology, namely a special nuclear magnetic resonance imaging analyzer with the magnetic field strength of lower than 1.0 Tesla is used for carrying out imaging analysis on a distribution condition of inner cracks of a rock coal sample; the sample is subjected to vacuum saturation water treatment and an image for reflecting a distribution condition of a rock core porosity is formed by signals generated by water in sample pores; an image resolution is higher and a recovery degree to an inner structure of the sample is better; and the method not only can accurately recover a pore environment inside the sample, but also can analyze and research a development process of sample cracks by means of repeatedly testing.