How can I do when I search some word from textbox in richTextBox, scroll being top on the searching word?

you need to use richTB1.ScrollToCaret() after changing the color of the selected word

In the example below,

Suppose textBox1.text="hello";

Click the search button each time, scroll button being top on the searching word

    int startindex = 0;
    private void Button1_Click(object sender, EventArgs e)
    {
        richTB1.SelectionBackColor = Color.Transparent;
        string word = textBox1.Text;
        int wordstartIndex = richTB1.Find(word, startindex, RichTextBoxFinds.None);
        if (wordstartIndex != -1)
        {
            richTB1.SelectionStart = wordstartIndex;
            richTB1.SelectionLength = word.Length;
            richTB1.SelectionBackColor = Color.GreenYellow;
            richTB1.ScrollToCaret();
            startindex += wordstartIndex + word.Length;
        }
        if (startindex >= richTB1.TextLength)
        {
            startindex = 0;
        }
    }

Try this extension

will Highlight a word in index line

 //richTextBox : your Control 
// word want to be Highlighted 
// index of Line 
// Color : Highlight  color 

 public static void HighlightWord(this RichTextBox   richTextBox, string word, int index, Color color)
    {
        richTextBox.SelectAll();
        richTextBox.SelectionBackColor = Color.White; 
        var lines = richTextBox.Lines;
        var start = richTextBox.GetFirstCharIndexFromLine(index);
        if (index < 0 || index >= lines.Length || start < 0)
            return;
        // Get the 1st char index of the appended text
        int indexword = lines[index].IndexOf(word);    
        var length = word.Length;//lines[index].Length;
        richTextBox.Select(start + indexword, length);               
        richTextBox.SelectionBackColor = color;
    }