Na-5　[B2P3O13]　(NBP)　is　a　promising　nonlinear　optical　crystal.　It　exhibits　a　wider　transparency　range　from　UV　to　far-IR.　And　its　absorption　edge　lies　at　186nm.　The　forbidden　band　gap　is　about　6.67eV.　The　time　domain　spectra　in　the　range　of　0.25-2.5　THz　were　measured.　The　power　absorption　coefficient　and　refractive　index　were　studied.　The　refractive　index　is　linear　with　frequency　from　0.4THz　to　1.8THz.　And　the　value　increases　slowly　from　1.8　to　2.1　THz.　The　properties　of　the　sample　show　that　it　is　possible　to　apply　it　to　optical　device.