The existing security analysis results for Piccolo attack only up to 14-round Piccolo-80 and 18-round Piccolo-128 with lower complexity than exhaustive analysis, except for biclique analysis. By analyzing the information disclosure law of the key-schedule algorithm of Piccolo, the related-key impossible differential cryptanalysis method is used to give the attack results on 15-round Piccolo-80 and 21-round Piccolo-128 with pre-whitening keys respectively. When 2⁸ related-keys are used, the data complexity of the attack is 2^58.6 and 2^62.3, the memory complexity is 2^60.6 and 2^64.3, and the computational complexity is 2⁷⁸ and 2^82.5 respectively. When 2⁴ related-keys are used, the data complexity, memory complexity, and computational complexity of attack are 2^62.6, 2^62.3; 2^64.6, 2^64.3; 2^77.93, 2^124.45 respectively. The analysis shows that the 15-round Piccolo-80 and 21-round Piccolo-128 with pre-whitening keys are insecure under the related-key impossible differential attack.