Abstract: Two types of unclosed staircase chart were cited, and reasonable assumptions were made on the test results that may appear during the staircase fatigue test. The effect of test data closed or not on the fatigue strength was analyzed when the fatigue strength was statistically estimated by the staircase method. The results show that when the last-level stress in the staircase chart of unclosed test data was lower than the first effective stress, the statistically estimated fatigue strength after subsequent supplementary of hypothetical staircase chart was also lower than the statistically estimated fatigue strength from the original unclosed fatigue test staircase chart. When the last-level stress of the staircase chart of unclosed test data was higher than the first effective stress, the statistically estimated fatigue strength after subsequent supplementary of hypothetical staircase chart was also higher than the statistically estimated fatigue strength from the original unclosed fatigue test staircase chart. Even if test data was not closed, the fatigue strength statistically estimated from failure events and non-failure events may be equal.