A method for red tide detecting and monitoring by the airborne blue-green lidar is proposed based on analyzing the blue-green optical properties of the seawater. The blue-green laser can enter the seawater deeply, and the information of red tide in the deep seawater can be detected by using the blue-green lidar. By detecting the blue-green light scattering signal under the seawater, the information of red tide density can be gained. A red tide density detecting model based on airborne blue-green lidar is established through analyzing the absorbing and scattering characteristics of the red tide. By emulating and computing, it is proved that the method can detect and monitor the density of the ocean red tide effectively, and therefore it can also predict the occurrence of red tide.