据mkl说, 我已经更换了
signature.FieldName = "Signature";
至
signature.FieldName = (pk.GetNameInfo(X509NameType.SimpleName, false)).Replace(' ', '_');
和
signatureAppearance.SetVisibleSignature("Signature1");
signatureAppearance.SetVisibleSignature((pk.GetNameInfo(X509NameType.SimpleName, false)).Replace(' ', '_'));
谢谢mkl。