The energy saving effect of low-pressure(LP) economizer of a 220MW unit was tested under multi-variable factors. Using the equivalent enthalpy drop method, the energy saving effect of LP economizer at different operational modes and different operating parameters has been calculated. When the higher water flow rate and inlet temperature of LP economizer is necessary, condensate water from NO.6 LP heater outlet should be used priority. Under the condition of 220MW, the optimal flow into LP economizer from NO.6 LP heater outlet is about 250t/h, the thermal efficiency of the unit could be increased 0.645%. If LP economizer inlet temperature is lower, the outlet of LP economizer should be connected to the outlet of NO.6 LP heater. And water temperature at LP economizer outlet should be slightly higher than the temperature of the NO.6 LP heater outlet. Otherwise, the energy saving effect will be decreased. Under the condition of 220MW, the thermal efficiency of unit increases 2.003%. For different LP economizer operation mode, there will be an optimal water returning location and optimal inlet flow rate.