Spermatogenesis is the process in which spermatogonia develop into mature spermatozoa. In most animals, this process involves changes in the basic protein associated with DNA. Somatic-type histones are partially or totally replaced by transition proteins, which in turn are replaced by protamines with strong alkalinity. With the replacement of basic proteins, nuclei progressively undergo chromatin condensation. The Chinese mitten crab (Eriocheir sinensis) sperm nucleus chromatin arrange loosely, and its formation is not clear. In this research, DNA sequence encoding histone H2A was cloned by PCR method. Thereafter, polyclonal antibody against the histone H2A was generated in rabbit, histone H2A distribution during E. sinensis spermatogenesis was detected through immunofluorescence. The encoding sequence of E. sinensis H2A contains 369 bp that encodes a protein of 123 amino acids. The predicted molecular weight of H2A was 13.1 ku. Protein alignment studies demonstrated that this histone H2A had a high identity with its counterparts in Penaeus vannmei and P.monodon, and the homology both are 99.19%. Immunofluorescence demonstrated that histone H2A was present through the spermatogenesis of E.sinensis. In spermatogonia and spermatocytes, H2A was mainly distributed in the nuclei and cytoplasm, while in spermatids and spermatozoa, H2A was mainly in the nuclei. The persistence of H2A in the mature sperm nucleus of E. sinensis might have some association with the uncondensed chromatin.