JUNA(Jinping Underground laboratory for Nuclear Astrophysics)is planing to measure the 13C(α,n)16O and some other important reactions at or close to stellar energies using intense helium beam out of an ECR driven accelerator in Jinping Underground laboratory[1].Deuterium impurity in ion source will produces significant amount of neutrons,limiting the background level.To control the deterium impurity,we have developed a method to measure the deuterium impurity within Helium beam using the d(d,p)t reaction.