The present invention discloses a key distribution method and system, the method includes: a card issuer management platform informing a supplementary security domain corresponding to an application provider of generating in a smart card a public/private key pair including a public key and a private key, receiving the public key returned from the supplementary security domain, importing a public key for trust point for external authentication into the supplementary security domain, and transmitting the information of the supplementary security domain and the public key to the application provider management platform; the application provider management platform receiving the information of the supplementary security domain and the public key from the card issuer management platform, and selecting the supplementary security domain of the smart card by a service terminal according to the information of the supplementary security domain and the public key; the application provider management platform informing the supplementary security domain of regenerating a public key and a private key, generating a supplementary security domain certificate according to the regenerated public key which is returned from the supplementary security domain, and achieving the supplementary security domain key distribution by transmitting the supplementary security domain certificate to the supplementary security domain. The present invention can improve the security of the supplementary security domain key distribution.