On the basis of thermal elastic mechanic equations, N-S equations and scalar wave equation, the thermal deformation of a reflector in a laser system caused by absorbing beam's energy and its effect on beam propagation were researched. Meanwhile, how much the blowing flow field would affect the beam at different blowing speeds was studied too. Then, the two effects were compared and their combined influence with sequential coupling on the beam was studied. Results show when the thin column reflector is free in constraint and is fixed at center point of back side, its deformation is rather small even radiated with a quite high power density laser beam. The effect of the deformation on beam propagation just displays as making beam tilted slightly in the outgoing direction and it is eliminated by pre-checking the optical system or is neglected directly. The results also show that blowing flow field will affect the beam quiet little when its speed is lower than 30 m/s. However, the effect will grow rapidly when the speed increases to rather high. Finally, it gives the conclusion that the combined influence on the beam is not evident when the blowing speed is not high and it could be omitted simply in most cases.