A flyback converter, a constant-current control method, and a lighting system are provided. The constant-current control method includes: sampling a loop current in a resonant loop to obtain a first sampling signal; obtaining zero crossing time of an excitation current in the resonant loop; and turning off a first switch tube when the first sampling signal reaches a reference value, and turning off a second switch tube after the zero crossing time of the excitation current in the resonant loop to control an average value of the excitation current to keep constant. The present disclosure can achieve a constant average value of an excitation current in each switching cycle based on a loop current in a resonant loop on the primary side of the converter, thereby realizing constant-current control of an output current of the converter.