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As passively mode-locked fiber lasers based on Nonlinear Polarization Rotation(NPR)have the problem of low stability and difficulty to self-start into a mode-locking state during operation,an online automatic mode-lock circuit was designed to adjust the laser in real time to get a stable modelocked output.In the design,the signal from NPR passively mode-locked laser was transformed to an electrical pulse signal with a high speed photoelectric detector,and it was amplified and shaped linearly to be received by the Micro Computer Unit(MCU)to count.Then the MCU distinguished the state of the laser based on quick mode-lock judge algorithm and adjusted the electric controlled polarization controller(PC)on the fiber laser automatically.The experiment results show that for the NPR modelocked fiber laser with a repetition rate of 6.238MHz,the automatic mode-lock circuit can distinguish the losing lock in 6ms,and can find and get into the mode-lock state in 10.24sautomatically.ForNPR passive mode-locked fiber lasers with repetition rate lower than 16 MHz,this automatic modelock circuit can get into a stable mode-locked state quickly or restart immediately if the mode-lock state is lost for some reasons,which reaches the design requirements and shows advantages of simple structures,low costs,low powers,and high reliability.