可使用以下代码：

//Create our own namespaces for the output
 XmlSerializerNamespaces ns = new XmlSerializerNamespaces ();  
//Add an empty namespace and empty value 
ns.Add ("", "");  
//Create the serializer 
XmlSerializer slz = new XmlSerializer (someType);  
//Serialize the object with our own namespaces (notice the overload) 
slz.Serialize (myXmlTextWriter, someObject, ns);

此外，在评论中还提到了去除开头的<?xml version="1.0" encoding="utf-8"?>的方法：

XmlWriterSettings settings = new XmlWriterSettings ();  
// Remove the <?xml version="1.0" encoding="utf-8"?> 
settings.OmitXmlDeclaration = true;  
XmlWriter writer = XmlWriter.Create ("output_file_name.xml", settings); 
  另外，如果出现开头没有encoding="utf-8"时，应该使用： 
 XmlWriterSettings settings = new XmlWriterSettings (); 
settings.Encoding = Encoding.UTF8;  
XmlWriter writer = XmlWriter.Create ("output_file_name.xml", settings);