Due to the extremely complex detection environment of empty area in deep underground engineering, the detection data is often limited, thus we can only obtain rough contours by conventional fitting method. In view of this, we propose a method for determining the 3-D profile of empty area by using borehole ultrasonic scanning technique. Firstly we present a preprocessing method for the effective screening of ultrasound scanning data by determining valid data points and smoothing non-effective detection points; secondly, by introducing the second-order Bezier curve, we present a curve fitting method which could cover every single effective detection point, favorable for the fitting characteristics of horizontal sections of the empty area; after horizontal contour fitting, we accomplishedvertical fitting by linear interpolation based on depth information, and acquired computation result and 3-D images by MatLab software. Subsequently we compared the results with those obtained from other fitting methods, and the comparison suggest that: 1) the borehole ultrasonic scanning system is able to provide effective data for the modeling of empty area; 2) the improved Bezier curve is feasible and suitable for the fitting of empty area profile; 3) the presented contour fitting method based on ultrasound scanning is proved feasible and accurate.