Organic aerosols exhibit a fluorescence effect, generating fluorescence when excited by laser at specific wavelengths. For such fluorescent aerosol clusters, a radial boundary detection algorithm based on horizontal detection fluorescence-Mie lidar is proposed. The algorithm employs echo signals from both fluorescence and Mie dual channels to create a function that eliminates atmospheric effects. Subsequently, adopting the intercept method to detect sudden changes of characteristic signals of fluorescent aerosol clusters, their boundaries can be determined. Numerical simulations demonstrate that the algorithm offers high accuracy and stability in detecting radial boundary for both single and double fluorescent aerosol clusters, regardless of visibility and signal-to-noise ratios. By utilizing this algorithm to detect the radial boundaries of single or double fluorescence aerosol clusters with fluorescence-Mie lidar, the accuracy and stability of boundary detection are significantly improved. Additionally, the algorithm is capable of timely detection of boundaries for single fluorescent aerosol clusters.