Downhole closed-loop drilling is the foundation of intelligent drilling. Currently, the closed-loop drilling system faces the challenge of downhole tools being unable to obtain depth information in real-time, especially in the slope-increasing section and slope-decreasing sections. Continuous monitoring of trajectory changes and real-time adjustment of the wellbore trajectory are required, and relying solely on downlink depth information can no longer meet the real-time depth information needs of closed-loop drilling. This paper proposes a method for measuring downhole depth in the slope-increasing section for the first time, laying the foundation for downhole tools to achieve closed-loop drilling. The specific method is: Two sets of directional sensors are deployed at a distance of 1 m from each other on the downhole tool. In the slope-increasing section, the sensors exhibit significant differences as the downhole depth changes. When real-time depth information of the downhole tool is required, the current depth is taken as the starting point (which can be obtained from the ground downlink), and the downhole control system collects the inclination curves of the two sets of sensors in real-time. The Euclidean distance between corresponding points of the two curves for each pair of sensors is calculated, and when the minimum value appears, it indicates that the depth of the corresponding tool has increased by 1 m. This process is repeated from the current moment to obtain a depth curve with an interval of 1 m. To verify the effectiveness of this method, two strictly identical sets of sensors spaced 1 m apart were installed on a section of drill collar, with storage capabilities. Experiments were conducted in a test well of CNOOC, and a well inclination-depth curve with a depth interval of 1 m was plotted. Two sections of well inclination data spaced 1 m apart on the curve were selected, and the depth corresponding to the build-up section was calculated using the Distance measurement method. Experimental data show that tests were conducted in the build-up section of a well section with a build-up rate of 5°/30 m, resulting in a maximum depth error of less than 0.5 m and a cumulative error of less than 0.44 m. As the build-up rate decreases, the depth error gradually increases. For well sections with smaller changes in well inclination, the depth calculation accuracy can be improved by increasing the interval between the two curves.