Abstract: The effects of the ratio d/σ of the stress step d to the standard deviation of fatigue strength σ, the number of stress steps i and the number of sample N on the average μ and standard deviation σ of fatigue strength measured by up-and-down method were studied through computer simulation. The results show that the reliability of fatigue strength average measured by up-and-down method is higher than that of standard deviation, and the reliability of test results can be improved by repeated measurements under the same test conditions. When d/σ>3, it is difficult for up-and-down picture to meet the requirement of D>0.3. The average and standard deviation of the estimated value of standard deviation σ are related to the number of stress steps i in up-and-down picture, and i should be 3 or 4 in up-and-down method test. It is useful to reduce the dispersion of estimated value (μ,σ) of (μ,σ) by increasing the number of specimens.