hanhphuc I would look at 'getattributes a bit closer as this does just that and in your code your doing it twice by using lee's method of getting by tag name there is no need for the OP's request.
an example
(setq lst '())
(setq ss (ssget (list (cons 0 "INSERT"))))
(repeat (setq x (sslength ss))
(if (/= (setq atts (vlax-invoke (vlax-ename->vla-object (ssname SS (setq x (- x 1)))) 'getattributes)) null)
(progn
(setq lst2 '())
(foreach att atts
(setq lst2 (cons (vla-get-textstring att) lst2))
)
(setq lst (cons lst2 lst))
)
)
)
(princ lst)