Simultaneous localization and mapping based augmented reality (AR) is trending in mobile AR applications. With the help of depth sensors, both accuracy and speed have improved. However, the method that performs the initial registration to align virtual objects with real scenery is not well developed, especially for some applications requiring very accurate registration (e.g., systems used in medical applications). For the iRay system, which is a mobile AR system using the Structure Sensor, we propose to use an iterative closest point algorithm to initially register the scanned mesh with the torso surface obtained pre-operatively, and then use SLAM to track pose. In this paper, a semi-automatic initial registration strategy is evaluated by a user study. This strategy is designed to help the user modify the selection of the 3D scanning area, so that the errors introduced by subjective differences can be reduced. The results indicate that the proposed strategy helps the users improve initial registration quality and reduces the average needed time.