The present invention relates to a constant gradient field nuclear magnetic resonance (NMR) rock sample analysis method and instrument. The method includes: in a constant gradient magnetic field, performing NMR measurement to acquire data; converting the measured NMR data into a two-dimensional NMR spectrum D-T2; performing measurement and inversion on a standard sample of a constant gradient field to obtain a standard sample two-dimensional NMR spectrum D-T2; measuring the sample to acquire a two-dimensional NMR spectrum D-T2 of a fluid in the sample; identifying fluid types according to the practically measured two-dimensional NMR spectrum D-T2; computing the fluid property and the petrophysical parameters according to the two-dimensional NMR spectrum D-T2 of the fluid in the sample; performing single slice scanning on the sample to acquire partial oil and water saturation; performing continuous slice scanning to obtain axial oil and water saturation distribution and movable fluid saturation distribution of the sample.