He was a great realist, writing about ordinary men and women in their misfortunes.

As he wrote about ordinary men and women in their misfortunes, he was a great realist.

参考译文：他是一个伟大的现实主义者, 写了许多平凡/普通的不幸中的人/男女.

writing 短语是什么成分？原因状语吗？

解答如下：

你的理解为正确，现在分词作原因状语，相当于上面原因状语从句

write about 写…的事，写到…，把（某事）作为写作的主题 

She is most powerful and touching when she writes about the student movement in the old days. 

她过去写的学生运动最有力动人。 

In the book he wrote about the matter. 

在这本书里, 他写到了这件事。