Ultrasound (US) image 3-D calibration for freehand probe is an extremely important technique in computerize 3-D ultrasonic (US image model reconstruction).Calibration is a procedure to calculate the spatial relationship between the US image and the tracker attached to the US probe. In this research, a different cross-string phantom and the corresponding algorithm are presented. The phantom with a set of crosses accelerates interactive operation speed through the strings and crosses out of the scanning plane guiding the operator quickly to find the scanning plane. The other, the ten crosses in the scanning plane provide the coordinates and spatial vectors for the calibration algorithm, thus the calibration algorithm can be optimized based on the least-squares fitting method of the homologous points matching. The results show that the scanning plane positioning time is no more than 5s. The precision and the accuracy demonstrate that the algorithm calculates more accurate matrix than that is obtained through other ways in the same operation time..