There is a point near Kyzyl, in the Tuva Autonomous Republic of Russia, with a monument erected to announce the fact that it is the geograpic centre of Asia. So presumably this is further from the sea than Urumqi.
But what is meant by "the sea" ? Do you count the Aral Sea as a sea ? If so, this will affect the answer. Anyway, the Aral Sea will probably have dried up completely in 50 years' time, so I think it can be safely ignored.
I'm revisiting this question after about 2 hours, because I've just looked it up in an atlas and it seems that Urumqi is alot closer to the mark than Kyzyl. By using a map of Asia and triangulating from the southermost point of the Gulf of Taz (a branch of the Gulf of the Ob), the westernmost point of the Gulf of Pohai, the northernmost point of the Bay of Bengal and the easternmost point of the Gulf of KaraBogaz (an arm of the Caspian Sea), I've a found a position 86·8° east, 44·8° north, that is about 2600km from the nearest sea; it's 265km NW of Urumqi and 50km NE of a small town called Manass. So Manass is the answer. However, the atlas is dated 1960 and the name may have been Sinified by now (i.e. become known by its Chinese name).
book: 'Tuva or Bust' by Richard Feynman
The Graphic Atlas of the World by John Bartholomew, 11th edition (1960)