To realize the energy-saving retrofit of low temperature economizers in the electrostatic precipitator area, the flow field in flue of the system was simulated by FLUENT 6.3. The results indicate that the flow field before precipitator appears extremely nonuniform, there are obvious high speed area and vortexes area in the deflection of the flue, the local maximum speed is up to 32m/s., the flow field is uneven in the both of the low temperature economizers after they are installed, and the maximum velocity deviation reaches 79.3%, which influences the heat transfer effect in the low temperature economizers greatly. There happens abrasion and scour in the low temperature economizers' pipes, which would endanger the safety of low temperature economizers. There appears large area at low velocity in the precipitator inlet, therefore the flow field is inordinate in the precipitator area, the precipitator power consumption is high and the collection efficiency is low. Optimizing the flow field and eliminating the vortexes can solve the precipitator efficiency problem. The results can provide reference for the engineering reform.