Abstract: To investigate the influence of welding electric parameters, heat transfer conditions on detection and control of weld penetration in gas tungsten arc welding process, a new mathematical model of welding electric parameters and latent heat, backside weld width in GTAW process was established by considering latent heat varying with temperature for 304 stainless steel. The effect of welding current and arc voltage on weld penetration was simulated, and the time of complete penetration was predicted under constant welding current in spot welding. To verify the accuracy and effectiveness of established model, a series of experiments were conducted and the laser dots images reflecting the dynamic variation of weld penetration were captured. After statistically analyzing and calculating the variable time of reflected dots shape from the automated frame images, the actual time from partial penetration to complete penetration was obtained. The experimental and simulated results show that the established mathematical model can respond the change of weld penetration with varying welding electric parameters, and the effect of welding current is more apparent. The predicted results based on the established model agree well with the measurement.