Abstract: Based on the 3D model of Guangzhou main built-up area at 1:7000 horizontal scale, 1:500,1:1000,1:2000 vertical scale respectively, a wind tunnel simulation is carried out on the neutral flow of B atmospheric boundary layer. With wind velocities in more than 26 test points,15 heights of each prevailed wind of the northwest and southeast, the uncertainties generated by reference height zref, zero-plane displacement height d and sample thickness t to exponent α and fitting precision are all discussed, which provide some guidance to raise fitting precision on power law applications later. It is founded that a reference height equal to maximum height of test points has a relatively smaller mean value and dispersion of α, and a better fitting precision than other zref ;d can improve fitting precision and is correlated with α negatively. The exponential law of d in the zero plane displacement is not considered; it is more practical especially for the researches of inhomogeneous terrain and numerous test points. In differently rough terrain, α converges at 0.25-1H of the thickness of wind profiles and 0.25-1H is the ideal sampling thickness.