He has been plying his trade as a carpenter in this small town for thirty years.

plying 是动词 ply 的现在分词形式。ply 在这里的意思是"从事、经营"，特指持续性地从事某种职业或手艺。ply one's trade 是一个固定搭配，表示"从事某人的手艺/职业"。