MORUS is a third-round candidate authenticated cipher of CAESAR designed by H. Wu et al. Dynamic cube attack presented by Itai Dinur et al. recovers the secret key of a cryptosystem by exploiting distinguishers given by cube testers. This study proposes an improved dynamic cube attack by optimizing standard of choosing cubes and method of recovering secret expressions. Based on which, a technique is presented for recovering crucial secret expressions preferentially with high successful rate. Then, MORUS of reduced to 5 steps is attacked utilizing improved dynamic cube attack, and the key of 128-bit is recovered with data complexity of O(2^95.05) and the successful rate of 92% at least.