A new seam-tracking system based on stripe type laser sensor and welding robot is proposed. The laser sensor is fixed to the end of the robot. The weld propfile data is abstracted in real- time during the welding process. And the wavelet transform modulus maxima theory is used to extract the welding characteristic- points. The contour data is grouped based on the characteristic- points. And then, the least square method is used to fit the weld lines and calculate the intersection point of straight line, which is aimed to determine the coordinates of the characteristic- points accurately. At last, the coordinate transformation calculation is used to get the trajectory of the welding torch in real-time. Then, two experimental verifications for different V- shape welds are carried on. Comparing experiments between only using wavelet transform modulus maxima theory and using wavelet transform modulus maxima and the least square method are performed. The experimental results show that the error of the former is about 0.514 mm, but the trajectory exists serious jitter phenomenon. The error of the other is about 0.304 mm and the jitter phenomenon significantly diminishes, which satisfies the requirements of automatic welding of requirements within 0.5 mm.