Abstract: In order to study the influence of the molten pool size on the burn-through during in-service welding,this paper investigated the radial deformation variation law with the change of internal pressure for different molten pools under specific temperature field with commercial software code ANSYS. The results show that when the temperature field remained constant and the internal pressure in the pipe was low,the value of radial deformation of the node,which had the highest temperature on inner surface of the pipe,was negative for one given molten pool. With the increase of pressure,the radial deformation of the node became positive and the radial deflection increased linearly. However,when the pressure exceeded a certain value,the linear change vanished and the radial deformation increased severely. For different molten pool sizes,when the penetration and pressure remained constant,the less the width and length of molten pool,the higher the instability temperature. When the penetration and the highest temperature on the inner surface of the pipe remained unchanged,the less the width and length of molten pool,the higher the instability pressure.