Abstract: To realize the visualization of mould processes and to assist the breakout prediction technology, a contour region filling method was proposed to plot an intuitive on-line thermal map of a mould quickly and efficiently. This method uses discrete data gridding, contour line generating and contour polygon filling to plot the nephogram. A combination of the grid sequence method with the cell subdivision method is adopted for contour line generating, which is more efficient, accurate and simple because of avoiding the contour line tracking and ambiguity. The method was compared with the scan parent element method. The results indicate that the former can plot the nephogram rapidly, ensure its precision and quality, so is better than the latter. Algorithm complexity analysis also proves that the time efficiency of the former is higher. Practical application results show that the on-line thermal map based on the method can reflect the temperature field characteristics of the mould in normal and abnormal conditions well, and is satisfactory for the process of practical production.