Crypton is one of the candidates of AES that designed based on Square which is a SP-network block cipher. Crypton attracts much attention of the world because of its excellent performance on hardware. The security of Crypton block cipher under impossible differential attack was studied in this paper. The properties of the diffusion layer and nonlinear layer of Crypton are analyzed and combined with the quick sort technique, the divide-and-conquer strategy, the early abort technique, the impossible differential attack on 7-round Crypton is improved with a lower data complexity and time complexity. By using 4 impossible differential distinguishers in parallel, combined with the property of key schedule, the master key of 7-round Crypton is recovered. Based on the impossible differential attack on 7-round Crypton, one more round is extended to maintain the attack on 8-round Crypton-256 to recover the 256-bit key with a data complexity of 2\+103 chosen plaintexts, a time complexity of 2\+214 8-round encryptions, a memory complexity of 2\+154.4 B. The results show that with the usage of several techniques and the properties of Crypton, the best impossible differential attacks on Crypton are proposed in this paper known before. These techniques can also be used to analyze the other SP-network block ciphers.