at last, this paper discusses 3-valued logic, points out some deficiencies of 3-valued logic system, and establishes a propositional logic system with 4 values : true, false, divergent and meaningless . then this paper proves that this 4-valued logic system is truth-functionally complete, and expands its connective 最后,本文对经典的三值逻辑进行了总结,在此基础上提出了一个四值逻辑系统,扩充了该系统的连接词,并讨论了该系统的一些代数性质。