To study the change rule of transverse relaxation characteristics and cross-linking density of hydroxyl-terminated polybutadiene(HTPB) liner under the condition of constant strain storage aging, accelerated aging tests of HTPB liner at 0%, 5%, 10% and 15% constant strain levels were designed. The proton transverse relaxation decay curves of HTPB liner were measured by the low-field ¹H NMR CPMG sequence and the cross-linking density of HTPB liner was calculated and obtained based on the modified single chain model. The change rule of cross-linking density was analyzed. At the same time, a cross-linking density aging model considering physical stretching and chemical aging factors was established and the influence factors and changing rule of the parameters of aging model were studied. The results show that the cross-linking density of HTPB liner shows a trend of increasing firstly and then decreasing under the condition of constant strain storage aging. The existence of constant strain causes an increase of the stress relaxation time of HTPB liner, but it has no significant effect on the aging reaction rate constant of HTPB liner. The specific form of established cross-linking density aging model is . The correlation coefficient R is more than 0.9500 (R>0.9000 at 5% constant strain) between the model and the experimental results, which can be used to describe the aging law of crosslink density of HTPB liner under constant strain conditions.