Abstract: According to JIS Z2275-1978, the plane bending fatigue test of FB780MP hot rolled steel plate was carried out. The least square method was used to fit the test data, and the fatigue empirical formula under symmetrical cyclic stress was obtained. The results show that the fatigue limit of the FB780MP hot rolled steel plate was 377.6 MPa under loading frequency 25 Hz and cyclic load ratio R=-1. The fatigue empirical formula fitted by least square method was σ=-36.56lnN+852.11. No inclusions or processing defects were found in the crack source zone of fatigue samples, and the characteristics of fracture surface morphology were in accordance with the set test conditions.