The key schedule algorithm plays a crucial role in the block cipher, thus many attacks pay more attention to it at present. Through analyzing the non-linear function FI and the key schedule algorithm of MISTY1, and observing the distribution of subkeys as well, a weak-key class of MISTY1 was found, which encompasses 2⁹⁰weak keys that are vulnerable to related-key amplified boomerang attack. Based on the weak-key class, two related-key differential characteristics were found. Then, the research presented a 7-round related-key amplified boomerang distinguisher of MISTY1, which can accomplish an attack on the 8-round MISTY1 without the last FL lay. The attack requires 2⁶³chosen plaintexts, and the time complexity of the attack is 2⁷⁰. The attack is the first attack on MISTY1 without the last FL lay. Besides, compared with the similar attacks, the limit of the related-key of our attack is released.