We developed an effective isolation method of magnesium lithospermate B from Salviae miltiorrhizae Radix and found for the first time that magnesium lithospermate B shows strong in vitro inhibition (IC50=0.04 μM) of aldose reductase (AR), 2.5 times than that of clinically used epalrestat (IC50=0.1 μM) and accumulation of fibronectin dose dependantly.