Quantum secure direct communication (QSDC) transmits secret messages directly over a quantum channel without the prior distribution of a key. Here, we apply Wyner's wiretap channel theory to analyze the security of QSDC protocols. The ideal protocol is treated as the main channel, and the effect of eavesdropping is treated as the wiretap channel. Entanglement‐based QSDC protocols are analyzed in detail at first. We calculated the channel capacity of the wiretap channel, and hence, the secrecy channel capacity of the protocol. The security of single‐photon–based QSDC protocols is studied through the equivalence between the entanglement‐based protocols and single‐photon–based protocols. We present a modified version of the single‐photon‐based DL04 protocol, which gives a higher secrecy capacity.