His bright smile was proclaiming his joy without words.

proclaiming 是动词 proclaim 的现在分词形式。proclaim 由前缀 pro-（表示"向前、公开"）和 claim（声称、宣告）组成，意思是"公开宣告、明确显示"。proclaiming 在这里表示"通过笑容明显地传达出"喜悦之情。