The ultrasonic cavitation threshold is a significant area of research in therapeutic ultrasound. This study conducts a numerical simulation of the ultrasound cavitation threshold by solving the dynamic equations of bubble clusters composed of bubbles with varying sizes. The effects of different criteria for cavitation threshold, bubble dynamics models, medium types, viscoelasticity and number of bubbles on the cavitation threshold are analyzed. Moreover, a comparison is made between calculation outcomes and those from previous experimental research. The results show that the relationship between the cavitation threshold P and frequency f can be expressed as P = Af + B, where A, B and α depend on the properties of the medium and different criteria of cavitation threshold. As the number of bubbles increases, the cavitation threshold initially rises and then falls, indicating a non-monotonic trend. Furthermore, the influence of the medium's shear modulus on the cavitation threshold is more intricate, and the cavitation threshold in viscoelastic medium is not consistently higher than that in water. Among all parameters in this study, the cavitation threshold is more sensitive to the change of frequency.