The effects of side ratio and incoming flow turbulence characteristics on base moment coefficients of rectangular high-rise buildings were investigated. Synchronization pressure wind tunnel tests for high-rise buildings with side ratios ranging from 1/9 to 9 under four terrain categories were conducted. The wind tunnel test results were compared with the data in previous literature. The effects of side ratio, turbulence intensity, and turbulence integral length scale on mean and standard deviation values of base moment coefficients in along-wind direction and standard deviation values of base moment coefficients in across-wind and torsional directions were analyzed. Results show that when the side ratio was not larger than 3, the mean and standard deviation values of base moment coefficients in along-wind direction first increased and then decreased with the side ratio. When the side ratio was larger than 3, the mean and standard deviation values of base moment coefficients in along-wind direction did not change with the side ratio. The standard deviation values of base moment coefficients in across-wind and torsional directions increased monotonically with increasing side ratio; the growth rate of the values in across-wind direction gradually decreased, while that in torsional direction increased with increasing side ratio. For the base moment coefficient in along-wind direction, increasing the turbulence integral length scale would increase the mean and standard deviation values, while increasing the turbulence intensity would decrease the mean value and increase the standard deviation value. The effects of turbulence integral length scale and turbulence intensity on the standard deviation values of base moment coefficients in across-wind and torsional directions varied with side ratio. On the basis of the analysis results, the fitted formulas for the base moment coefficients in the along-wind, across-wind, and torsional directions of the rectangular high-rise building under different terrain categories were proposed, which can provide reference for structural design and load code revision.