Cannot receive correct message from my device?

I tried to send a long message （about 2000 Bytes, not ASCII encoded string, it is a sensor data package）to my app, then I translated the String object to Uint8List which received in listen function: (String line) {
Uint8List data = Uint8List.fromlist(line.codeunits);
total_data.addAll(data);
}
when the transmission started I cleared the total_data, and when the transmission finnished, I found that the length of total_data was only 1200, where is the problem? And how can I receive the raw data instead of String object?