Abstract: In this paper,the basic characteristics of the model spin-spin relaxation has been discussed.Via series expansion method,the Bloch equations controlled by diffusion effect have been solved for sphere pore space geometry saturated with oil and water.From the result,it is found the NMR relaxation is controlled by multi relaxation modes,and associated with pore size,surface relaxation ability,diffusion coefficient of pore fluid and oil saturation.Numerical results have shown that for water-oil two-phase saturated pore space,when pore size is small and as oil saturation increases,the water relaxation time decreases linearly.However,when pore size is relative large,the multi relaxation mode turns to be single mode with the increasing of oil saturation.And at the same time,the variation of relaxation time with oil saturation is non-linearity.