Abstract: Planetary boundary layer height (PBLH) is an important parameter in the study of weather, climate and atmospheric environment. However, there is no effective method to retrieve PBLH of the whole day based on lidar system. In this paper, the applicability and limitation of the Gradient method (GRD), the Standard Deviation method (STD), the Ideal Curve Fitting method (ICF) and the Wavelet Covariance Transform method (WCT) for the retrieval of PBLH are evaluated by using ceilometer data collected at Chaoyang and Daxing stations in Beijing. It is found that the GRD is susceptible to environmental noises and the ICF has preferable stability, but the latter would mistake residual layer height for PBLH in the nighttime when turbulence is weak. On this basis, this paper establishes a Two-step Ideal Curve Fitting method (2-S ICF), which divides the structure of planetary boundary layer into convective boundary layer in the daytime, and residual layer and stable boundary layer in the nighttime. Through two-step fitting with different ideal curves, the full-day PBLH can be obtained. The retrievals of 2-S ICF are compared with those obtained by the potential temperature gradient method based on L-band sounding radar. The results show that the correlation coefficient of results by the two methods is 0.91, which verifies the feasibility of the 2-S ICF and the application potential of the ceilometer in detecting PBLH. PBLHs at Chaoyang Station and Daxing Station from May to June 2017 are retrieved by using this method. It is found that convective boundary layer develops more rapidly in the daytime and the PBLH is higher at Chaoyang Station because of the influence of the special surface characteristics in urban area. The PBLH ranges are 308—1391 m and 197—1302 m throughout the day at Chaoyang station and Daxing station, respectively.