To reveal the mechanism of the resistance of the forebody and afterbody during stage separation at low altitude, the interstage flow field during axisymmetric body stage separation was numerically simulated. Two typical structures of interstage flow field and the transition characters between two structures were presented. The regulation of the resistance was investigated. With the relative distance increasing, the flow field of the interstage region changes from the vortex dominant flow to shock wave dominant flow. When the oblique shock wave appears before head of the afterbody, the resistance of the afterbody has a step increment and the influence of the afterbody on forebody is completely cut off. The increase of the afterbody's resistance in a distance mainly results from the enhancement of the oblique shock wave. Therefore it's still much smaller than the resistance in the independent flight and even smaller than the forebody. On the other hand, the method which employs the resistance of the whole body before separation and the forebody after separation to calculate the resistance of afterbody at the beginning of separation is proven to be reasonable and val-id at the end of the paper.%为了揭示级间冷分离过程中前、后体阻力变化的机理，对低空超音速条件下轴对称弹体级间冷分离流场变化过程进行了数值模拟，得到了级间区域的2种典型流场结构及2种流场结构过渡阶段的演变特征，得到了前后两体阻力特性的变化规律。随着轴向相对距离的增加，级间流场结构由涡流主导向激波主导逐渐转变。在后体头部脱体斜激波形成时，后体阻力出现突增，后体对前体的影响基本隔绝。但在一段距离内，后体阻力的增加主要来源于头部脱体斜激波的增强，导致后体阻力仍远小于其单独飞行时的阻力，甚至小于前体阻力。另外，证明了采用分离前全弹阻力与分离后前体阻力之差计算分离初始后体阻力是合理且保守的。
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