Lidar is a powerful tool for atmospheric monitoring. However, most lidars operate only in nighttime due to the interference of strong background skylight in daytime. We have developed the AML-1 lidar system, which can measure aerosol profiles in daytime also. In this paper, the structure and specifications of the lidar system are described. Various noise sources of the lidar and methods for rejecting these noises are discussed. It has been found that among these noises the sky radiation is the main background noise for daytime working lidars. This noise can be effectively eliminated by incorporating small divergence-angel of emitting laser beam, narrow-field-of-view receiver, and narrow-band filter with high transmittance. Some daytime boundary layer aerosol profiles measured by the AML-1 are presented.