In order to solve the problem that no basis can be followed in selection of instrument isolation valves in full containment LNG storage tank top and cryogenic valves are used conservatively in practical engineering design, conducted is analytical investigation on functions and operation and maintenance processes of such instrument isolation valves and structural requirements of LNG medium to crγogenic valves in codes, standards and specifications. Meanwhile, considered are design parameters of LNG storage tank and nozzle design in LNG storage tank top, conducted is constrastive analysis on cryogenic valve structure and found out is the theoretical basis for selecting instrument isolation valves and such conclusion is drawn that carbon steel valves or austenitic stainless steel valves can be used as instrument isolation valves in LNG storage tank top and it is not necessary to use cryogenic valves. Successful application of carbon steel valves or austenitic stainless steel valves in Tianjin floating LNG project shows that such selection is reasonable and much cost can be saved and procurement cycle can be shortened.