AERONET (Aerosol Robotic Network) level 2.0 and CM_21 data in SACOL (Semi-Arid Climate and Environment Observatory of Lanzhou University) for 2006-2012 were used to analyze optical properties and radiative effects of aerosols over Northwest China. Meanwhile, the reason for the positive TOA (Top of Atmosphere) radiative forcing was examined by using the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer). The annual averages of BOA (Bottom of Atmosphere), TOA, and atmosphere radiative forcing are -59.43 W m^-2, -17.03 W m^-2, and 42.40 W m^-2, respectively. The annual averages of AOD (aerosol optical depth, 550 nm) and α (angstrom exponent, 440-675 nm) are 0.37 and 0.91, respectively. α and AOD have opposite phases. The α value is within 0.0-0.2, which is very small, when the AOD is within 0.3-2.2. The annual averages of SSA (single scattering albedo, 675 nm), g (asymmetry factor, 675 nm) and the real part of the complex refractive index (675 nm) are 0.93, 0.68, 1.48, respectively. The annual trend of the real part is consistent with that of AOD while that of the imaginary part is opposite to that of AOD. Thereby the coarse-particle mode aerosols that have strong scattering effects dominate Northwest China. The maximum atmospheric heating rate occurs in 0-2 km and decreases with height based on the simulations with and without aerosol effects. The heating rates in the winter half year and summer half year are 2.6 K d^-1 and 0.6 K d^-1 on the surface, respectively. In the four seasons, the heating rate is the largest in the winter, followed by that in the autumn and spring, and smallest in the summer with values of 2.5 K d^-1, 1.4 K d^-1, 1.2 K d^-1, 0.2 K d^-1 on the surface in the winter, autumn, spring, and summer, respectively. The absorption by aerosols is stronger in the autumn than in the spring. Contributions of the surface albedo and SSA account for 22.5% and 77.5% of the positive radiative forcing at TOA, respectively.