A method for creating a multi-namespace includes steps of: returning information of a namespace data structure according to a query command from, wherein the information of the namespace data structure comprises a maximum number and a total capacity of supportable namespace; receiving and determining whether a create command for creating a plurality of namespaces is correct, wherein the create command comprises a number of a namespace and a capacity of the namespace; and if the determination is correct, creating a global host logical-flash physical address (H2F) mapping table according to the create command, wherein a number of the global H2F mapping tables is independent of the maximum number of the supportable namespaces and the number of namespace. A method for accessing data in a multi-namespace is also provided.