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In order to transmit the secure messages,a deterministic secure quantum direct communication protocol,called the Ping-pong protocol was proposed by Bostrtom and Felbinger [Phys.Rev.Lett.89,187902 (2002)].But the protocol was proved to have many vulnerabilities,and can be attacked by eavesdroppers.To overcome the problem,an improved security detection strategy which inserts the |O>,|1>,|+> and |-> particles into the messages as the decoy particles randomly in the Ping-pong protocol is presented.During the security analysis,the method of the entropy theory is introduced,and three detection strategies are compared quantitatively by using the constraint between the information which eavesdroppers can obtain and the interference introduced.Because of the presence of the trap particles ｜+> and |->,the detection rote will be no less than 25% when Eve attacks the communication..The security analysis result shows that the efficiency of eavesdropping detection in the presented protocol is higher than the other two,so the detection strategy in the protocol can ensure that the Ping-pong protocol is more secure.