Abstract: The product characteristics of gas field with rich H2S and CO2natural gas were analyzed，the structure of low alloyed steel overlaid by alloy 625 for well separator was determined and the proper welding process was selected．The weldability of alloy 625 was analyzed，and the proper welding process parameters were found out based on the process experience．Three overlaying procedures of one layer electroslag welding，shielded metal arc welding and automatic argon welding inside of small diameter pipe were qualified．Besides，the evaluation tests for intergranular corrosion，magnesium chloride stress corrosion and H2S stress corrosion resistance and dynamic corrosion under simulated working condition were carried out．The results are acceptable and meet the technical requirements．The equipment fabrication was completed．