The invention relates to a nuclear magnetic resonance rock sample analysis technology for petroleum geology, which is a gradient permanent magnet for a constant gradient field rock sample analyzer. The gradient permanent magnet comprises a yoke plate, magnetic steel, a polar plate and a side yoke plate, wherein the yoke plate and the side yoke plate form a closed frame structure; the magnetic steel is connected with the yoke plate; the polar plate and a magnet polar plate connected with the magnetic steel adopt soft iron with a trapezoidal section; the ratio of a long bottom edge to a short bottom edge of the trapezoidal section of the gradient polar plate is 5:130 to 5:100; the angle alpha of the two polar plates is 10-30 degrees; and an air gap L formed between the two polar plates is 150mm-364mm. The invention solves difficult problems which are not solved for a long time, no additional gradient generating equipment is added during measuring, and thus a magnetic field with a constant gradient can be generated to meet a physical environment for self diffusion coefficient measuring of formation fluid molecules, the measuring cost is low, and the fault rate of instruments is reduced.