Password protected cryptosystem is commonly adopted nowadays. The main shortcoming of such cryptosystem is the lack of the connection between the user and password. The special biometric protected cryptosystem adopting biometric key scheme, which binds the biometric template and cryptographic key monolithically, can be a good solution to the problem. The biometric key scheme is devised in such a way that the cryptographic key cannot be retrieved without a successful biometric authentication. A novel biometric key scheme based on modular secret sharing is proposed in this paper. The feasibility and high security have been proved by theoretical analysis and experiments. In the experiment of combining iris recognition with the scheme, the probability for illegitimate user to retrieve the cryptographic key is zero; the probability for legal user fail to retrieve the cryptographic key is 0.0571. The optimal biometric key is 2645 times more difficult for an attacker to retrieve the cryptographic key comparing to the legitimate user.