The broadband yellow luminescence(BYL) of undoped GaN film is fitted optimally by the linear superposition of multiple Gaussian functions by using nonlinear optimization technique. It is found that the BYL may be divided into the three Gauss_type spectra. It shows for the first time that the BYL originates from the combination of three independent radiative transitions. The energy levels of the initial states of the three independent transitions are measured directly in experiment by means of a novel absorption_normalized photoluminescent excitation spectroscopy. It is found that the three independent transitions have the same initial state but different final states. The origins of the initial and final states are assigned reasonably.