Earth′s field nuclear magnetic resonance (EF NMR)approach is used to study the spin relaxation process of protons in water. Two different spin relaxation times，T1 and T2，are measured to be (2.46±0.16)s and (0.83±0.02)s，which are in well agreement with the results T1 = 2.3±0.1 s from the reference [1]. Besides，the experimental results also confirm the Curie Law. The measurements on copper sulphate solution imply that the existence of ions in solution will accelerate the spin relaxation process. This work provides a way to study spin relaxation process in seconds scale by EF NMR.