A portable computer and security operating method thereof are provided to secure the portable computer in a power-off mode. The portable mainly includes a computer host, a system BIOS (Basic Input/Output System), an EC (Embedded Controller), an EC BIOS, one or more input device and a power device, all of which are in bus connection or circuit connection with each other. The EC includes a KBC (Keyboard Controller) and remains activated when the portable computer is in the power-off mode. Meanwhile, the EC is adapted so that in the power-off mode of the portable computer and upon receipt of a security signal, the EC executes a power-off-mode security routine of the EC BIOS to lock the portable computer in the power-off mode.