An　automatically　mode-locked　fiber　laser　based　on　the　nonlinear　polarization　rotation(NPR)　effect　was　demonstrated　and　a　stable　mode-locked　pulse　train　was　obtained.　A　model　of　parameters　such　as　transmittance,　self-phase　modulation　and　self-amplitude　modulation　of　the　NPR　mode-locked　fiber　laser　was　established.　The　relationship　between　the　equivalent　saturable　absorption　effect　and　the　rotation　angles　of　the　waveplates　was　analyzed.　Using　that,　an　automatic　mode-locked　fiber　laser　based　on　NPR　method　and　hill-climbing　algorithm　was　built,　and　the　mode-locked　output　could　be　obtained.　Output　characteristics　were　discussed　also.