The invention discloses a system and method for transforming and upgrading the electric output adjusting capacity based on steam processes. The system comprises a boiler, a high-pressure cylinder, anintermediate-pressure cylinder, a low-pressure cylinder, a first adjusting cooling device, a second adjusting cooling device, a third adjusting valve and a heat network heat supply system. According to the system, the first adjusting cooling device, the second adjusting cooling device and the third adjusting valve are additionally arranged in the main steam process, the reheat steam process and the low-pressure cylinder inflow steam process of a heat supply unit, furthermore, the flow directions of main steam, reheat steam and low-pressure cylinder inflow steam are changed through the first adjusting cooling device, the second adjusting cooling device and the third adjusting valve, a proper adjusting method is adopted, the flow of inflow steam entering the high-pressure cylinder, the intermediate-pressure cylinder and the low-pressure cylinder is reduced, the power capacity of the cylinders is reduced, it is achieved that the power capacity of the high-pressure cylinder, the intermediate-pressure cylinder and the low-pressure cylinder is reduced to zero to the minimum degree, the electric output of the heat supply unit is reduced, and the electric output adjusting capacity of the heat supply unit is improved.