In resent years, the accounts about adults' sudden death without specific reason have been increased. The definition of cause of death was a hot potato to legal medical experts. With the deep - going of etiology to molecular level, clinical cardiac diseases' researches have confirmed that the arrhythmia has been resulted from a kind of " idiopathic disorder of cardiac action potential" that is related to cardiac sodium channel diseases by SCN5A mutation confirmed by using the techniques of molecular genetics. The paper reviewed the characteristic of SMDS ( Sudden Manhood Death Syndrome) in forensic science and some kinds of diseases' genotype and phenotype in clinical medicine, and hoped to acquire some revelation for further related research.