The invention relates to a method and production line to manufacturing an insulating tank for a liquefied natural gas carrier. The method includes the working procedures of ''mould piggybacking'', ''clapboard assembling'', ''temporary clapboard fixing'', ''clapboard fixing by nailing'', ''roof plate assembling and temporary fixing'', ''roof plate fixing by nailing'', ''bar code affixing'', ''tank body overtuning'', ''double-layer roof plate assembling and temporary fixing'', ''double-layer roof plate fixing by nailing'', ''tank body leveling up'', ''insulating material filling'', ''filling density checking'', ''bottom plate assembling and temporary fixing'', ''bottom plate fixing by nailing'', ''tank body finishing'', ''tank body conveying and cleaning'', ''tank body packaging'' and ''tank body transporting''. By implementing such technical process for producing insulating tanks applied to LNG ships, the labor is largely saved, the production efficiency is improved, and the requirement of high precision of insulation tanks is satisfied.