He stood there looking at her walking out of his sight.

句中looking at her walking out of his sight 是现在分词短语，在句中用作伴随状语。

在looking at her walking out of his sight 这个现在分词短语中， walking out of his sight是looking at的宾语（her）补足语。look at sb./sth. doing sth. 的意思是“看着某人/某物正在做某事”，例如：Last night his mother looked at him doing (his) homework. 昨天晚上他妈妈看着他做家庭作业。