Trojan horse attacking strategy on quantum cryptography is investigated. First, the fragility of the quantum cryptographic algorithm employing EPR (Einstein-Podosky-Rosen) pairs as a key against the Trojan horse attacking strategy is analyzed. To prevent the Trojan horse attacking, an improved scheme which makes use of the non-orthogonal entangled states is proposed. This scheme is robust to the Trojan horse attacking, without reducing the security on other kinds of attacking strategies.