哎。。。。 还是我来解决吧!!!
void TextEdit::SetLineBackColor(QColor color)
{
QTextCursor cursor = this->textCursor();
QTextCursor cursor1(this->document()->findBlock(cursor.position()));
QTextCharFormat plainFormat(cursor1.charFormat());
QTextCharFormat colorFormat;
colorFormat.setBackground(color);
cursor1.movePosition(QTextCursor::EndOfBlock,QTextCursor::KeepAnchor,1);
cursor1.mergeCharFormat(colorFormat);
}