Abstract: The foam volume for large-scaled oil tank is determined based on the 《Design Criterion of Lower-Multiple Foam System》 or on the protection perimeter of foam generator. However, during fire extinguishing process of large-scaled oil tank, the two calculation methods are too ideal to apply, which is only suitable for the rectangular design of the tank wall with foam dam baffle. In allusion to this question, a calculation method of foam volume for large-scaled oil tank is discussed and improvement on increasing fire extinguishing effectiveness is presented.