The types of porosity in weld of CO,2, arc welding can be classified into two categories（metallurgical reaction type and precipitation type）,the natures of the porosity are CO porosity,hydrogen porosity and nitrogen porosity respectively.The formation of porosity consists of three stages of nucleation,growth and escape of bubbles. The sensitivity of CO,2, arc welding to porosity is determined by the metallurgical properties of the process. Among the process influencing factors,Welding current,arc voltage,gas flow rate,power polarity and welding speed are the main factors that affect the porosity tendency. The inharmonious relationship between "small welding spatter and large porosity tendency" is related to the droplet transfer modes and the gas carrying behavior of the droplet. Strictly controlling the compositions of welding consumables,base metal and groove cleanliness are the necessary conditions to control the porosity in the weld,and adopting correct process parameters,auxiliary process and operation technology are the necessary conditions to control the porosity.