In the Formal Deductive System L* of Fuzzy Propositional Calculus, the notion of closed theory is introduced and its properties are investigated. Furthermore, the completeness of Formal Deductive System L* is proved through closed theory based on formula set F(S). At first, in the formal deductive system L*, a concept of closed theory is introduced, and a method for extending theories to closed theories is given; at second, in the formal deductive system L*, a concept of total closed theory is introduced, and the existence of a total closed theory satisfying relevant conditions is proved; at third, in the formal deductive system L*, the properties of congruence relations determined by closed theories are investigated, a concept of strong congruence relations is introduced to formulas set F(S), and methods of changing each other between strong congruence relations and closed theories are revealed; at fourth, in the formal deductive system L*, it is proved that closed theory style L*-Lindenbaum algebras determined by closed theories are R0-algebras, and a closed theory-L*-Lindenbaum algebra is linear if and only if a closed theory is total; at last, the completeness of the formal deductive system L* is accomplished by making use of total closed theory style L*-Lindenbaum algebras, and the results obtained before have been improved.