Abstract: This paper describes the general principle and experimental equipment of diagnosing are plasma by monowavelength pulsed laser holographic interferometry,The solution of the holographic interference patterns had been put forward and the free burning argon are diagnosed.The errors of diagnoses were discussed and the results verified.It is shown that the said technique and solution are effective and reliable.Wide application of this method is expected as it is particularly advantageous to the study of the are under non-normal pressure and the dynamic process of are plasma.