An algorithm was designed to assess the fluctuation in the mode control ability of the photonic lantern. This algorithm incorporated a module specifically for extracting the gray matrix from the light field image. To analyze the fluctuations in the photonic lantern's mode control along with the beam power， numerical results were used in place of angular power distribution variance. Initially， the theoretical foundation of the algorithm was established using the power flow equation and adjacent mode coupling theory. Following this， the gray extraction algorithm， detailing its structure and parameters were elaborated. By utilizing the gray matrix， a restored image was derived and compared with the original light field image. The findings confirmed that the algorithm effectively translates the light intensity distribution into a gray value matrix. Finally， the mode control changes of a custom 3×1 photonic lantern was evaluated under varying beam-combining power. During the experiment， the beam combining power was increased from 0 to 270 mW. The experimental outcomes indicate that our analysis accounts for the variations in the slope of the light beam combining loss curve for the 3×1 photonic lantern and the fluctuation in the maximum Gaussian fit degree of the combined beam as the power increases. In summary， this algorithm offers a simple and efficient method for assessing the fluctuation in the mode control ability of the photonic lantern when generating the fundamental mode beam. It exhibits low environmental sensitivity， and its accuracy in extracting light field data exceeds 99%.