The defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni2MnGa single crystal under a high magnetic field of 50 KOe applied along the [1 (1) over bar0] crystallographic direction of the Heusler phase were studied by the in-situ high-energy X-ray diffuse-scattering experiments on the high energy synchrotron beam line 11-ID-C of APS and thermomagnetization measurements. Our experiments show that a magnetic field of 50 KOe applied along the [1 (1) over bar0] direction of the parent Heusler phase can promote the premartensitic transition of Ni2MnGa single crystal, but puts off martensite transition and the reverse transition. The premartensitic transition temperature (T-PM) increases from 233 to 250 K (-40 to -23 degrees C). The martensite transition start temperature (M-s) decreases from 175 to 172 K (-98 to -101 degrees C), while the reverse transition start temperature (A(s)) increases from 186 to 189 K (-87 to -84 degrees C). The high magnetic field leads to a rapid rearrangement of martensite variants below the martensite transition finish temperature (M-f). The real transition process of Ni2MnGa single crystal under the high magnetic field was in-situ traced.