Abstract: Numerical model of temperature field and fluid flow field was built to study the effects of gravity on different welding positions and different gravity values. Temperature field and fluid flow field of flat position, 45° position and 90° position with downward speed were calculated, respectively. The results showed that fluid flow of 90° position was far more intense than the one of flat position, which affected the temperature distribution and caused molten metal flowing downward. The presence of gravity had no influence on fluid flow distribution morphology, but affected the flowing speed significantly. Conclusions of this study can be used on the research of welding pool instability judgment basis and deciding parameters of all position TIG or MAG welding.