The quality,safety and health risks of 9% Ni steel vertical weld by shielded manual arc welding for large LNG tank are mainly discussed. Through analysis and field construction experience,it is found that the causes of these hidden dangers are poor replicability of welding procedure,manual operation of welder,carbon arc gouging back chipping,extensive grinding,inadequate supervision,difficulty of high altitude operation,etc. It is suggested that using an efficient automatic AC GTAW welding procedure to replace the traditional shield manual arc welding. This suggested welding procedure has the characteristics of high quality,strong adaptab-ility,cleanliness and high cost performance.