You could take metro to Shanghai South Railway which operates many trains departing to Yiwu everyday. The journey will take you 3~6 hours. For quicker method, it is suggested that you take metro Line 2 from downtown area to Shanghai Hongqiao Railway Station, which has some high-spend trains to Yiwu, taking no more than 2 hours.

three options:1.take a coach from shanghai south coach station to Yiwu.take a shuttle coach from shanghai pudong airport to Yiwu directly. take a train from shanghai south railway station to yiwu.