There exists a direct relationship between the weld pool oscillation frequency and size of weld pool. But the detection of weld pool in continuous condition is difficult by using arc voltage and light. In order to detect the weld pool oscillation frequency in continuous welding,a new laser-vision method is proposed. Several experiments with partial,critical and full penetration for continuous welding are conducted and the frequency is detected by the image-processing algorithm. The characteristic of the weld pool oscillation frequency in different penetration status is analyzed by the shape variation of laser-line,surface oscillation of weld pool and internal flow. The results show that there are an oscillation mode in weld pool surface and an internal flow in partial and full penetration which showed single oscillation frequency. In critical penetration,pool oscillation shows distinct boundary which has the features of partial and full penetration and there are multiple oscillation frequency. This study provides a forceful foundation for accurate detection of different penetration status.