Could you please advise me the fastest way to travel from HK airport to Guangzhou Fair ground ?

Take the direct cross-border coach to Guangzhou Haizhu District. You may get off at Rosedale Hotel or Land Mark Canton Hotel. It takes around 3.5h. Then take a taxi to the Canton Fair in around 15min.

Or airport shuttle bus no. A21 to Hung Hom Railway Station within 60-70min. Take a through train to Guangzhou East in 2h. Then take a taxi to the fair site in 15min.