Reducing the flue gas temperature deviation is one of the characteristics of tower boiler，but the flue gas temperature deviation still exists in the actual operation，and there is still a lack of detailed research on its formation mechanism. In this paper，a 660 MW tangentially fired tower boiler was used to study the characteristics of flue gas flow field deflection and temperature deviation in the penal heating surface area under rated working conditions. Three cases with different flue structures and heating surface layout were designed and analyzed，and the smoke temperature deviation mechanism was deeply and carefully studied. The results show that the deviation characteristics of simulation are consistent with the experimental values and actual operation deviation characteristics，and there are obvious flow deflection and temperature deviation in the area of penal heating surface. The velocity and temperature in the left area were significantly higher than those in the right area. With the increase of height，the flue gas temperature deviation in the left and right region increases firstly and then decreases，and reaches the maximum near the entrance of the third-stage superheater. The mechanism of flow field deflection and gas temperature deviation is as follows:First，after the gas rotates and rises into the pipe penal，it is constrained by the pipe penal，and the velocity component perpendicular to the direction of the pipe penal turning around，causing gas near the left wall flowing to the forward wall，and gas near the right wall flowing to the back wall. Then，because the flue gas outlet at the top of the furnace is asymmetrically arranged on the rear wall，under the suction of the downstream induced draft fan，gas near the left wall first inclines to the forward wall，then turns to the back wall，and is more centered in the area of the penal heating surface. On the contrary，gas near the right wall is first inclined to the back wall，and then is drained away by the draft fan along the back wall，resulting in a serious deflection of the flow field and a large scale of reflux.