The invention discloses a spherical aberration compensating device in an optical storage system. The compensating device comprises a microprocessor used for adjusting the compensation value of a spherical aberration compensating driver for a plurality of times in an optical combined head after determining the category of optical storage medium and before the optical combined head achieves the focalization to the optical storage medium, and a digit signal processor used for processing a plurality of electric signals converted through the optical combined head after each time the compensation value of the spherical aberration compensating driver is adjusted by the microprocessor. The microprocessor allows the spherical aberration compensating driver to drive the spherical aberration compensating unit of the optical combined head according to the width values of focalization error signals, so as to compensate the spherical aberration of light spots caused by the optical combined head focalizing on the optical storage medium. The invention also discloses a spherical aberration compensating method. By adopting the device, a high-cost photosensor is not required to be built in the optical combined head, the influence by a disk sheet is avoided, and the spherical aberration of the light point caused by the optical combined head focalizing on the disk sheet can be accurately compensated.