In this paper, an in situ technique was used to determine the laser absorptivity of solid and liquid surfaces of Al alloys. The experimental results show that the surface condition of the sample is the most important factor affecting the laser absorptivity. Usually, the absorptivity increases a little with the increase in laser power. But if the laser power is too high, the absorptivity of blacked samples will be decreased because of burnout of blacking layer. When melting pool emerges on the surface of sample, the measured laser absorptivity is an ensemble average value for liquid and solid zones within the area of laser spot, and the real absorptivity of liquid surface will be determined by using a method combining experiment measurement and numerical simulation. It is also shown by the results that if the incident angle of laser beam is in a range from 0° to 45°, the absorptivity increases linearly with increasing the angle, whatever the surface of specimen is in a state of solid or liquid. The data processing procedure was also improved in this paper. The complex curvilinear equation was replaced by a linear equation and the experimental data were dealt with using linear regression method.