Abstract: In order to study the horizontal wind speed-up effect on two adjacent hills, computational fluid dynamics (CFD) simulation was employed and systematic analysis was made. The speed-up ratios of the two hills in parallel, tandem, and staggered arrangement were obtained and their distribution characteristics were investigated. The speed-up ratios of typical cases obtained from the CFD simulations were compared with those obtained from the wind tunnel tests in order to validate the present CFD simulation. Results show that, typical characteristics of horizontal wind speed-up ratio on a hill are that the front part, hill crest and rear part are the deceleration zone, the maximum acceleration position and the wake zone, respectively. The speed-up ratios of the two adjacent hills in tandem arrangement increase with the decrease of the hill distance and reach their maximum values when the hill distance is zero. The speed-up ratios of the isolated hill are less than those of the two adjacent hills with arbitrary hill distance, indicating that the isolated hill case is equivalent to the two hills with infinite hill distance. For two adjacent hills in gapless staggered arrangement, the front hill has shielding effect on the rear hill to some extend, and the existence of the rear hill slightly reduces the wind velocity of the front hill. Therefore, the speed-up ratios of the front hill are bigger than those on the rear hill and smaller than those on the isolated hill; however, the discrepancy between them is insignificant. The speed-up ratios of the hills obtained by the CFD simulation are close to those obtained from Australian and New Zealand Code, and the regulation in the Chinese Code regarding the horizontal speed-up ratio is relatively conservative.