A new lidar low-level wind shear alerting algorithm based on ramps detection was presented in this paper. Glide path scanning and combination of double and simple ramp detection had been applied in this new algorithm. Firstly, the lidar radial velocity was projected onto the glide-path and headwind wind profile was restricted into. Secondly, single ramps and the changes of double ramps declivity were detected by the new algorithm. The results on provisions of international low-level wind shear threshold and the wind shear alerting discriminating judgment formula was compared to come to the conclusion. The performance of the proposed method was verified through the real data from Hong Kong Observatory. The results show that the proposed algorithm detects the wind shear which the single ramp fails to detect. The new proposed method is effective, it is significant to raise the alarm rate.