How can i set the caret position to a specific index in passwordbox in WPF

You can try something like this to set the selection in the PasswordBox:

private void SetSelection(PasswordBox passwordBox, int start, int length) {
    passwordBox.GetType().GetMethod("Select", BindingFlags.Instance | BindingFlags.NonPublic).Invoke(passwordBox, new object[] { start, length });
}

After that, call it like this to set the cursor position:

// set the cursor position to 2...
SetSelection( passwordBox1, 2, 0);

// focus the control to update the selection
passwordBox1.Focus();