A topographic multiplier is widely used to evaluate the effect of topography on the design wind speed. The maximum value of directional topographic multipliers has been used as the topographic multiplier in many design codes. However, this simplified method is conservative and overestimates the speedup effect in some cases. In this paper a topographic multiplier considering wind directionality is proposed by using Monte Carlo simulation, and investigated at two typical meteorological stations. Then a reduction factor is proposed as the ratio of the topographic multiplier estimated by Monte Carlo simulation to that obtained by the simplified method. A topographic variation coefficient is proposed as the deviation of directional topographic multipliers and investigated systematically by using terrain models. As a result, the reduction factor decreased as the topographic variation increased. An empirical formula is proposed to improve the simplified method.