A method includes: receiving a first result obtained by performing BIP check on a sent first to-be-checked bit stream; performing BIP check on a received second to-be-checked bit stream to obtain a second result, where the second to-be-checked bit stream is a bit stream received by a receiving device after the first to-be-checked bit stream is transmitted; detecting a type of a control block in the second to-be-checked bit stream, and determining a third result based on impact of the type of the control block on a BIP check result; comparing the first result, the second result, and the third result; and if the first result is different from the second result, the first result is different from the third result, and the second result is different from a predetermined result, determining that a bit error occurs when the first to-be-checked bit stream is transmitted.