A falling protective device for protecting a hard disk of a falling portable computer against damages includes a falling sensor arranged in the portable computer for generating and sending an interrupt signal to a keyboard controller in response to a detected falling state of the computer. An SMI signal line is extended between and connected to the keyboard controller and a system BIOS of the computer. On receipt of the interrupt signal generated by the falling sensor, the keyboard controller sends an SMI signal via the SMI signal line to the system BIOS, which in turn sends a park control signal to park the hard disk or a power-off control signal to terminate the supply of working power to the hard disk. The system BIOS sends a polling signal via a polling signal line to the keyboard controller, so as to poll about a state signal of a default status bit in a default signal port of the keyboard controller.