The invention discloses a supplementary settlement method for electrical energy by adopting a reduction method. The supplementary settlement method comprises the following steps of: (1) establishing a supplementary settlement parameter model in the system server of an electricity trade centre, and setting supplementary settlement parameters; (2) establishing a reduction environment; (3) amending a supplementary settlement environment; (4) performing a supplementary settlement by adopting a reduction method; (5) issuing a supplementary settlement result, generating a settlement list, and transmitting the settlement list to an electricity plant; (6) processing the supplementary settlement result by the electricity plant; and (7) performing settlement execution tracking by the electricity plant. Via the supplementary settlement method disclosed by the invention, the supplementary settlement caused by various reasons such as electricity quantity missing, turning for new machines to commercial service, operation for desulfuration and denitration equipment and the like, and the faults of thermoelectric data channels is overcome, the refined management granularity of settlement is greatly increased, the problem of the electricity quantity execution tracking and analysis brought by the supplementary settlement is effectively solved, and accurate tracking and analysis for the electricity quantity execution condition of the electricity plant are realized.