DN gas fields are located in the Dongqiulitage structure belt in Kuqa depression. Currently, three gas reservoirs, DN1, DN2 and DN3, have been discovered with 3P reserves of more than 2 000×10⁸m ³ in the reservoirs. More gas are expected to be discovered. With the successful application of varying velocity mapping technology, we have estimated the DN2 gas field and discovered the DN3 gas reservoir. Interval velocity is the key factor in discovering traps. This paper discusses the method about varying velocity mapping used in estimating the DN2 gas field and discovering the DN3 gas reservoir. The good method to discover or estimate traps is that the prestack depth migration technology is used in 3D work area and the interval velocity filling is used in 2D work area.