Abstract: The purpose of this paper is to studythewall jet in defined space duringtunnel construction. For two-dimensional wall jets in defined space, through a lot of experiments and theoretical analyses it is con- cluded that: (1) the flowfield ofwall jet in defined space can be categorized as thewall jet zone, the back flow zone and the eddy zone; (2) the boundaryof awall jet can be approximately regarded as a straight line and the angle of spread is 7.67 degrees; (3) the flowmotions in the developing region and fully developed region of the flow are similar; and (4) along the distance, the maximumvelocity of profile decays faster in defined space than that in half-limitless space; and the length of the back flowzone in a defined space wall jet increases with the increase of the pipe flow. In this paper, the formula for calculating the length of the back flow zone is presented.