Massive MIMO provides a high spatial degree of freedom (DoF) to support multiple users. To gain the spatial DoF, accurate channel information is crucial, and the pilot is important to obtain it. However, in grant-free transmissions, different users may select the same pilot, which leads to pilot collisions. To solve this problem, this paper proposes an irregular superimposed multi-pilot. The proposed superimposed structure provides a large multi-pilot pool than the conventional cascaded one. Using this structure, the number of single-pilots in multi-pilot can be different, and the pilot number distribution is optimized via a novel Monte-Carlo-based differential evolution. The simulation results verify the analysis and show that the proposed method outperforms existing works.