Under Sun Ce's service, he slew the Shanyin bandit leaders Huang Longluo and Zhou Bo , earning him several ranks and a thousand more soldiers under his command. He later followed Sun Ce on his solidification campaigns throughout the region.
When Sun Ce was assassinated in 200, his mother Lady Wu was worried that his successor Sun Quan would be too young to govern the region. She consulted Dong Xi about the area's security, and Dong Xi replied, "The lands east of the Yangzi have security from mountains and rivers, while the good government and virtue of Sun Ce have already attracted the people. Sun Quan can build on these foundations, so that great and small follow his commands. Zhang Zhao can look after affairs of government, while others like me act as claws and teeth. With both strategic advantage and popular support, there is certainly nothing to worry about." And the others agreed with him.
Later, the bandits of Poyang rebelled with over ten thousand men, Dong Xi was dispatched with Ling Tong, Jiang Qin and Bu Zhi to suppress them. Wherever Dong Xi went, the rebels under Peng Hu fled, and the rebellion was dealt with in ten days.
In 208, Sun Quan led a punitive expedition against Huang Zu. Huang Zu set two ships covered with ox-hide to guard the channel across Miankou , and he had a great rope of coir-palm fibre, with stones attached as anchors. Above all this were a thousand crossbow men to give covering fire. The arrows poured down like rain and the army could not get forward. Dong Xi along with Ling Tong were together in the van, each in command of a hundred volunteers in double armour. They boarded a great barge, charged the covered ships, and Dong Xi cut the two ropes with his sword, clearing the blockade. The main body of the army then advanced. Huang Zu escaped through a gate but was pursued and beheaded by enemy troops. At a great gathering the next day, Sun Quan raised his goblet to Dong Xi saying, “Today’s gathering is to celebrate the achievement of the one who cut the two ropes!”
At the Battle of Ruxukou against Cao Cao's great army, Sun Quan sent Dong Xi to command the “five-storied ship” and station at the mouth of Ruxu. During the night, there was a large storm and the “five-storied ship” was about to capsize. His subordinates scattered and fled and pleaded for Dong Xi to come out, and Dong Xi replied sternly, “As generals who have received responsibility to get ready to face the rebels, how can anyone flee? Anyone who dares utter this will be beheaded!” And so no one dared to persuade him. Finally, the ship foundered and Dong Xi went down with the ship.