As for natural gas miscible flooding in low-permeability reservoirs, studying on the phase behavior characteristic of oil–gas miscible zone has a significant meaning. In this paper, based on the reservoir characteristics of Donghe 1CIII oilfield, the evaluation of phase physical properties and minimum miscible pressure was conducted by laboratory experiments using recombined formation fluid and natural gas samples. Then, one compositional model was established to analyze the phase behavior of the miscible zone during natural gas flooding. The result shows that the fluid critical points of the miscible zone change the polynomial in the P–T diagram. More specially, the curve of critical points owns more area with higher pressure, which means that the fluid reaches a deeper miscible degree. Finally, a new chart method for gas channeling identification is proposed to evaluate the production state of the miscible zone in order to identify gas channeling. The phase envelope of the production fluid is concerned with the migration of the miscible zone. In addition, an application case in Donghe oilfield is given by means of the gas channeling-phase diagram identification method. This work could provide a theoretical basis for timely gas channeling prevention and control measures as well as improvement of the gas injection development effect.