Abstract: Waste heat boiler (WHB) is important equipment recovering waste heat of the flue gas in industry, the recovery efficiency of the WHB is closely related with its internal temperature field distribution. The ISA WHB has a three-passage structure（rising passage-descending passage-horizontal passage）, and exhaust gas, after entering from the descending passage, flows underneath the convection banks in the horizontal passage, under the effect of inertia, which has an adverse effect on heat exchange of the horizontal passage. With the help of the simulation method, a flow guide plate was installed around the corner to lead the gas to convection banks. The results show that with the help of the plate, the distribution of velocity in the horizontal passage tends to uniform: the range from 2.0 m/s to 3.5 m/s is expanded, and the maximum and minimum of the velocity are both contracted. The flow rate of enthalpy through convection banks rises from 5.27 MJ to 7.70 MJ, while the flow rate of enthalpy through underneath the banks declines from 4.89 MJ to 2.90 MJ. At the same time, the district of low temperature caused by backflow is narrowed, which is beneficial to low temperature corrosion.