In　this　paper,　the　crystallization　and　relaxation　of　NaCl　aqueous　solutions　at　low　temperatures　were　studied　by　differential　scanning　calorimetry　(DSC)　analysis　and　dielectric　analysis.　From　DSC　analysis,　the　three-phase　composite　ice　structural　model　consisting　of　pure　ice,　salty　ice,　and　amorphous　ice　after　the　liquid-solid　phase　transition　of　NaCl　aqueous　solution　under　low　temperature　is　established.　The　conversion　rate　of　pure　ice　decreases　monotonically　with　the　increase　of　solution　concentration.　From　dielectric　analysis,　the　relaxation　time　of　water　molecules　in　composite　ice　decreases　significantly　at　first　and　then　increases　slightly　with　increasing　concentration,　while　the　relaxation　activation　energy　increases　significantly　at　first　and　then　decreases　slightly.　This　is　due　to　the　damaging　effect　of　ions　on　hydrogen　bonds　between　water　molecules　and　the　interaction　between　anions　and　cations.　This　study　can　provide　theoretical　support　and　experimental　basis　for　cryopreservation　technology　of　biomaterials　controlled　by　electromagnetic　field.　©　2019　IEEE.