Abstract: The Richtmyer-Meshkov(RM) instability of a N₂/SF₆ interface subjected to a perturbed shock is investigated experimentally in a vertical shock tube. The perturbed shock is generated by a planar shock diffracting around a rigid cylinder and the initial uniform interface is formed by a membraneless method. Three different dimensionless distances η (the ratio of spacing from the cylinder to the interface over the cylinder diameter) are considered. Dynamic images of the interface evolution after the impact of the reflected shock are obtained using both schlieren and planar Mie scattering techniques. Our previous study (Zou, et al., 2017) indicated that, after the impingement of the incident shock, the interface evolves into a "Λ" shape structure with two interface steps at both sides and a cavity at the center. The results in present paper show that, due to the impingement of the reflected shock, the "Λ" shape structure interface first experiences a fast phase reversal and then the perturbation increases gradually. For η=2.0 case, the interface evolves into an overall bubble structure, while for η=3.3 and η=4.0 cases, a spike appears in the center of the interface besides the overall bubble. The mixing width is further measured from Mie scattering images and compared with the theoretical values. It is found that at the linear stage, the interface width can be predicted well by the linear model proposed by Meyer and Blewett, and at the nonlinear stage, the width can be reasonably estimated by the model proposed by Dimonte and Ramaprabhu. In particular, the distinction between the theoretical prediction and the experimental result is the lowest for the case of η=4.0.