Accurate measurement of the 3D shape of the dynamic weld pool is of great importance for on-line welding process control and off-line welding phenomenon study. However, it remains challenging due to the following reasons. Firstly, the weld pool is dynamic and liquid with extremely high temperature, which requires a nondestructive single-shot measurement technique. Secondly, the specular surface and the ambient strong arc light prevent direct imaging by cameras. Here, we present a structured light technique that yields closed form solutions with femtometre accuracy to measure the dynamic shape of the weld pool produced by gas tungsten arc welding (GTAW). This technique makes use of the band-pass filter and the high penetrability of lasers to overcome the glaring arc light. The sampling points on the weld pool surface are measured by computing the intersections of the projected laser rays and the corresponding reflected laser rays analytically. A conformal mapping transforms the physical world into the virtual world to make the measurement free of optics aberrations. As a result, the achieved measurement accuracy is up to 10(-13) mm, which is significantly superior to state of art methods.