This study presents a new thought to explain the mechanism of asymmetric flow in the boiler furnace with a symmetrical structure by extracting results of the numerical simulation of 3D flow in the boiler furnace with symmetrical structure. As an example of the thought, the numerical simulation results of 2D flow model show that whether using laminar model or turbulence model, the numerical results of flow in the furnace have asymmetric phenomenon within some parameter range. With laminar model, when Re of burner nozzle is less than 1500, the velocity field is symmetrical distribution; when Re reached 2500, the velocity field is asymmetric; while Re reached after 4000, the velocity field in the furnace is obviously asymmetric, the static bifurcation appears and the instantaneous value of velocity is oscillating over time. With turbulence model, when Re is less than 4000, the average speed by time is time-independent and symmetry; When Re is greater than or equal to 8000, the velocity field is asymmetric, which means the average speed by time also has oscillation. The numerical simulation results of 3D flow model show that the foursquare tangential circle tends to deviation given a symmetrical boundary condition. The results are illustrated to show that, the cause of the asymmetrical flow field is the internal reason for a flow problem with a completely symmetrical structure which provides a new thought of the mechanism of thermal deviation appears in the boiler furnace.