在传递给FormData之前对嵌套的json进行编码。它将工作。例如：

String horseJson = json.encode({"name": "Abisynia", "breed": "demo", "sex": "male"});
String ownerJson = json.encode({
          "personal_data": "John Doe",
          "email": "asasdasss@gmail.com",
          "phone_number": "3556234432"
        });
String breakfastJson = json.encode({"name": "morning", "manufacturer": "Adasa", "amount": 1});
String dinnerJson = json.encode({"name": "noon", "manufacturer": "Aasd", "amount": 2});
String supperJson = json.encode({"name": "evening", "manufacturer": "ASds", "amount": 3});
String questiinnaireJson = json.encode({
        "horse": horseJson,
        "owner": ownerJson,
        "breakfast": breakfastJson,
        "dinner": dinnerJson,
        "supper": supperJson
      });
FormData formData = FormData.fromMap({
      "questionnaire": questiinnaireJson
    });
var response = await dio.post(url, data: formData, options: Options(headers:_headers));

要使用FormData发送JSON数据，需要将其作为MultipartFile发送，并将内容类型指定为"application/json"。
就像这样：

final data = FormData.fromMap({
    "file": await MultipartFile.fromFile(
      path,
      filename: name,
    ),
    "jsonData": MultipartFile.fromString(
      jsonEncode({"name": "user_name"}),
      contentType: MediaType.parse('application/json'),
    ),
  },
);

经过几个小时的搜索，我解决了它。
如果你想发布单个嵌套的Map，可以说：

• • 错误：**

FormData.fromMap({

 parent_key: { child_key: child_value }

})

您需要将其转换为：

• • 正确**

FormData.fromMap({

 parent_key.child_key: child_value

})

如果你想发布Map列表，让我们说：

• • 错误：**

FormData.fromMap({

 parent_key: [

   { child_key: child_value },
   { child_key: child_value_02 }
]

})

您需要将其转换为：

• • 正确**

FormData.fromMap({

 parent_key[0]child_key: child_value
 parent_key[1]child_key: child_value_02

})