A non-invasive positioning system for determining the focus location of a HIFU device comprises a diagnostic ultrasound and the HIFU for ablating and removing tumor tissue. The imaging plane of the diagnostic ultrasound probe and the geometrical axis of a probe of the HIFU define an inclining angle during operation. When the imaging plane of the diagnostic ultrasound intersected to the focus of the HIFU transducer, a maximal convergent interference signals was obtained, so as to position the HIFU focus within tumors for precise ablation.