A dynamic reliability moment-independent importance measure index for double random vibration systems, in which the randomness of structural parameters and excitation is taken into account, is introduced to analyze the effect of the input variables on the dynamic reliability. Based on the expressions of the unconditional dynamic reliability and conditional probability density function (PDF), the importance measure index is numerically obtained. The index can effectively describe the mean effect of the PDF on the dynamic reliability from the distribution density of the basic variable. Then, on the basis of the state dependent parameter (SDP) model, the SDP method is established to estimate the defined moment-independent importance measure index. Additionally, two examples are given to illustrate the advantages of the presented method. Compared with the direct Monte Carlo method, the results show that the established solution can considerably improve computational efficiency with acceptable precision and be suitable for non-linear dynamic reliability responses of double random vibration systems.