这样做:

TextField(
  decoration: new InputDecoration.collapsed(
    hintText: 'Username'
  ),
),

或者如果你需要其他东西，如icon，设置边界InputBorder.none

InputDecoration(
    border: InputBorder.none,
    hintText: 'Username',
  ),
),

将焦点边框更改为none

TextField(
      decoration: new InputDecoration(
          border: InputBorder.none,
          focusedBorder: InputBorder.none,
          contentPadding: EdgeInsets.only(left: 15, bottom: 11, top: 11, right: 15),
          hintText: 'Subject'
      ),
    ),

decoration: InputDecoration(
 border:OutLineInputBorder(
 borderSide:BorderSide.none
 bordeRadius: BordeRadius.circular(20.0)
 )
)

试试下面的代码:

TextFormField(
  decoration: InputDecoration(
    border: OutlineInputBorder(borderSide: BorderSide.none, borderRadius: BorderRadius.circular(30.0)),
    hintText: "Search your trips",
    hintStyle: const TextStyle(color: Colors.white, fontWeight: FontWeight.w300),
    filled: true,
    fillColor: Colors.cyan[200],
    suffixIcon: IconButton(
      onPressed: () {},
      icon: const Icon(Icons.search, color: Colors.white),
    ),
  ),
),

新的Flutter SDK，因为在集成了web和桌面支持之后，你需要像这样单独指定:

TextFormField(
  cursorColor: Colors.black,
  keyboardType: inputType,
  decoration: InputDecoration(
    border: InputBorder.none,
    focusedBorder: InputBorder.none,
    enabledBorder: InputBorder.none,
    errorBorder: InputBorder.none,
    disabledBorder: InputBorder.none,
    contentPadding:
      EdgeInsets.only(left: 15, bottom: 11, top: 11, right: 15),
    hintText: "Hint here"),
)

下面是一个补充答案，显示了一些更完整的代码:

  Container(
    decoration: BoxDecoration(
      color: Colors.tealAccent,
      borderRadius:  BorderRadius.circular(32),
    ),
    child: TextField(
      decoration: InputDecoration(
        hintStyle: TextStyle(fontSize: 17),
        hintText: 'Search your trips',
        suffixIcon: Icon(Icons.search),
        border: InputBorder.none,
        contentPadding: EdgeInsets.all(20),
      ),
    ),
  ),

注:

深色背景(代码未显示)是Colors.teal。 InputDecoration也有一个fill和fillColor属性，但是我不能让它们有一个角半径，所以我用一个容器代替。