Disclosed is an inter-card channel protection method in a wave-length division multiplexing system. At the transmitting end, signals from the client side are divided via a coupler into two paths which are delivered to two cards, respectively. The two paths of signal transmitted by the two cards are sent via different routes to the receiving end. At the receiving end, the two paths of signal enter two cards, respectively, and the two cards communicate with each other directly. The standby card submits the signal quality detected by a detecting device to the primary card. The switching control unit on the primary card issues the commands of switching on/off the client-side lasers to the standby card in accordance with a decision made. One of the client-side lasers on the primary card and standby card is switched on while the other is switched off. The merits of this method include: protection can be realized with failures of card or essential chips thereon; protection switching can still be conducted with failures of system control unit; and, direct transfer of switching conditions and switching commands between the two cards is faster than communications with a system control unit via mailbox respectively.