Hard disk consumes about 20% of power of a portable system. In order to reduce the power consumption and prolong the service time of portable systems, a method of dynamic power management for hard disk, including the hard-disk state conversion by reading and writing hard disk registers, the load acquisition by reading interrelated kernel structures, and the design and implementation of high-performance policy optimization algorithms, is devised. Experimental results indicate that the dynamic power management of hard disk helps to significantly reduce the power consumption of portable systems, and that the proposed management method is feasible.