The atmospheric boundary layer can be directly influenced by the ground and it is closely related to human activities, so the detection and investigation of the atmospheric boundary layer is very important. Due to the abundant rainfall in the plum rain season in southern China, the atmospheric boundary layer height (ABLH) is very different from any other time of the year. Lidar is an active remote-sensing instrument, and the advantage of high spatial and temporal resolution makes it very suitable for the detection of the atmosphere. In this paper, a three-dimensional (3D) polarized lidar is introduced and the structure will be given in detail. Compared to traditional one-direction ground-based lidar, the pointing of the 3D scanning lidar is very flexible and can be adjusted to any direction within the up hemisphere (360 degrees by 90 degrees) in a very short time. The ABLH in the plum rain season (from June to July 2016) over Hangzhou area (30°16′ N, 120°07′ E) was observed and different derivation methods, such as the wavelet covariance method, the gradient method, and the profile fitting method were carried out and compared in detail. The results show that the wavelet covariance method exhibits better stability than the gradient method and better accuracy than the profile fitting method. This work brings a more flexible and accuracy way for the ABLH detection and will be of great importance to the atmospheric study during the plum rain season.