The invention discloses a multi-low-pressure-cylinder systematic optimization device for a steam turbine generator unit. The multi-low-pressure-cylinder systematic optimization device for the steam turbine generator unit comprises a steam turbine high-pressure cylinder, a steam turbine medium-pressure cylinder, steam turbine low-pressure cylinders and a low-pressure cylinder steam inlet valve, wherein the low-pressure cylinder steam inlet valve is mounted on a communicating pipeline between the steam turbine medium-pressure cylinder and the steam turbine low-pressure cylinders. The invention further discloses an optimization method for the steam turbine generator unit. According to the optimization method, operating parameters of the steam turbine generator unit are monitored in real time through a steam turbine generator unit control system, the low-pressure cylinder steam inlet valve is closed automatically once the steam turbine generator unit control system detects that the load of the steam turbine generator unit is lower than a set value, and the low-pressure cylinder steam inlet valve can be automatically opened completely if the load of the steam turbine generator unit is higher than the set value. By adoption of the technical scheme, when the steam turbine generator unit operates at a low-load operation state, the steam inlet parameters of the steam inlet low-pressure cylinders are improved by removing one low-pressure cylinder in operation, the air blast loss of the steam inlet low-pressure cylinders is reduced, the operating efficiency of the steam inlet low-pressure cylinders is improved, the operating stability of the unit is improved, and defects of the prior art are overcome.