Mcl-1 amplification has been observed in breast cancer and demonstrated as a key determinant of breast cancer cell survival. However, the clinical use of available effective Mcl-1-specific inhibitors for breast cancer treatment remains a challenge. An RNA-guided CRISPR/Cas13a system targeting RNAs can be used to specifically knock down mRNA expression in mammalian cells. The goal of this work is to develop a self-degradable nanoplatform based on polylysine (PLL)-functionalized black phosphorus (PBP) for the delivery of Cas13a/crRNA complexes to specifically inhibit Mcl-1 at transcriptional level for breast cancer therapy. The constructed Cas13a/crRNA complex is delivered into the cytoplasm by PBP via endocytosis, followed by endosomal escape based on the biodegradation of PBP, and this efficiently knocks down the specific gene at transcriptional level up to an efficiency of 58.64%. Through designing CRISPR RNA crMcl-1, Mcl-1 can be specifically knocked down at transcriptional level in breast cancer cells, resulting in the down-regulation of the expression of Mcl-1 protein and inhibition of the cell activity. Notably, PBP/Cas13a/crMcl-1 shows an excellent tumor suppression efficacy up to 65.16% after intratumoral injection. Therefore, biodegradable PBP is an ideal nanoplatform for the delivery of CRISPR/Cas13a, which could provide a potential strategy for gene therapy.