阐述了用 Rossi-α法测量瞬发中子衰减常数α的基本原理、测试方法及条件。用 BF3计数管作为探测器 ,采用时间延迟符合的测量方法 ,测量得出了实验室快临界装置在缓发临界时的瞬发中子衰减常数 αc,其值为 0 .835( 1± 0 .5% ) μs-1,并在次缓发临界以下 0～ 1＄次临界度范围内测得多个点的 α值 ,通过外推再次得到 αc,二者在误差范围内一致。最后给出 :αc=0 .835± 0 .0 0 4 μs-1 The basic principle, test methods and conditions of instantaneous neutron decay constant α measured by Rossi-α method are expounded. Using the BF3 counter tube as the detector, the instantaneous neutron decay constant αc of the fast critical device in the laboratory was measured by time delay coincidence measurement and its value was 0.835 (1 ± 0. 5%) μs-1, and the α values ​​of multiple points were measured within the critical range of 0 to 1 $ below the sub-slowing threshold, αc was obtained again by extrapolation, and the two were within the error range. Finally gives: αc = 0 .835 ± 0. 0 0 4 μs-1