reading json file with chinese character using streamReader

I have a json text file with chinese character.
the character is shown perfectly in notepad++ when I chosse the encoding as UTF8.
Hence I set the streamReader's encoding as UTF8 too.
But the character is not read correctly and log as some unknown character.

 public class SpeechManager :MonoBehaviour {
     
     public string fileName;
     public string speechJson;
     public int startId;
     public int endId;
     
     public Dictionary<int, Speech> speechDictionary=new Dictionary<int, Speech>(); 
     
     
     void Start(){
         getSpeechData ();
         putAllSpeechToDictionary ();
         getSpeechByID (100001);
         getSpeechByID (100002);
         getSpeechByID (100003);
 
     }
     
     
     
     public void getSpeechData(){
         try
         {
             string fileFullPath= Application.dataPath+"/textFiles/"+fileName;
             StreamReader theReader = new StreamReader(fileFullPath, Encoding.UTF8);
             speechJson=theReader.ReadToEnd();
             theReader.Close();
             
         }
         catch (Exception e)
         {
             Debug.Log(e);
         }
     }
     
     public void putAllSpeechToDictionary(){
         var N = JSON.Parse (speechJson);
         Debug.Log ("試試 "+N.Count);   // the chinese character print perfectly here
         for(int i=0;i<N.Count;i++){
             Debug.Log (N[i]);    // but failed here
             int id = N[i]["ChatID"].AsInt;
             int headiconID= N[i]["HeadiconID"].AsInt;
             int chatRole= N[i]["ChatRole"].AsInt;
             int chatstring= N[i]["ChatString"].AsInt;
             int triggerType= N[i]["TriggerType"].AsInt;
             int triggerScene= N[i]["TriggerScene"].AsInt;
             int triggerPic= N[i]["TriggerPic"].AsInt;
             int nextChatID= N[i]["NextChat"].AsInt;
             string speechCharacter= N[i]["SpeechCharacter"];
             string speechString= N[i]["SpeechString"];
             Speech tempSpeech= new Speech(id, headiconID,chatRole,chatstring,triggerType,triggerScene,triggerPic,nextChatID,speechCharacter,speechString);
             Debug.Log ("Adding speech "+ id);
             speechDictionary.Add(id, tempSpeech);
         }
     }
     
     public void getSpeechByID(int speechID){
         Debug.Log ("getting speech"+ speechID);
         Speech speechReturn= speechDictionary[speechID];
         Debug.Log (speechReturn.ToString ());
     }
 }