In order to transmit secure messages, a quantum secure direct communication protocol based on extended three-particle GHZ state was presented, in which the extended three-particle GHZ state was used to detect eave sdroppers. In 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 eavesdroppers can obtain and the interference introduced. If the eavesdroppers intend to obtain all information, the detection rate of the original "Ping-pong" protocol is 50%; the second protocol used two particles of EPR pair as detection particles is also 50%; while the presented protocol is 58%. At last, the security of the proposed protocol is discussed. The analysis results indicate that the protocol in this paper is more secure than the other two.
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