Ultrasonically activated surgical devices (UASDs, ultrasonic scalpels) are widely used in surgery, cutting soft tissues through direct mechanical, cavitation, and thermal effects. However, these damage effects and their contributions to cutting efficiency have not been systematically studied. This paper aims to experimentally identify the contributions of the cutting effects of UASDs to cutting efficiency and tissue damages. First, isolation experiments for the three effects were designed to investigate the conditions and forms of damage. Second, controlled cutting experiments are designed and cutting efficiency evaluation method are proposed to determine the contributions of these effects to cutting efficiency. Third, an experimental platform for mechanical-thermal measuring, capable of multi-axis motion of UASDs, is constructed while the above experiments are conducted. Experiment results found that the cavitation effect and the direct mechanical effect are the dominant contributors among these three effects to cutting efficiency in UASDs for soft tissue. During cutting process, direct mechanical effect only caused tissue removal at the contact location. The cavitation effect significantly enhanced the damage region beyond the contact volume, and the damage region is consistent with the intensity of the cavitation effect. UASDs cut at limited temperatures and thermal effect contributed insignificantly to cutting. Our findings suggest that cavitation effect, rather than thermal effect, is the key factor for UASDs' high-efficiency in cutting soft tissues, which can help to optimize the UASDs tool and provide a basis for detailed modeling of UASDs.