Natural draft dry cooling systems (NDDCs) are essential cold-end equipment for power generating unit. The theory of NDDCs utilize the difference density between air flow through the heat exchanger to take away the heat for maintaining the system operation. Its performance is easily affected by environmental conditions,under crosswind conditions,the effectiveness of the NDDCs is significantly reduced due to the disturbance of the flow field uniformity in the tower. The current predominant approach is based on "try and error" method to find the position of the deflector which can reduce turbulence, this approach mainly determined by experience without any theoretical support to fill this gap. This paper uses the flow field synergy principle, which aims to reduce momentum dissipation, as a theoretical guide to optimise baffle installation based on the theory. The momentum equation is modified for declining viscous dissipation by means of the variational method, using the modified momentum equations taken into the three-dimensional model to obtain optimum flow field. According to guidance of the flow field, the shape of the deflector and the position are determined. In the presence of crosswind, the pressure drop at the inlet and outlet of NDDCs is dramatically reduced 60.54% compared to previous optimised model based identical flow rates. The results indicated significant reduction in flow resistance and energy consumption after optimization.