Fragility of quantum private communication based on Einstein-Podosky-Rosen (EPR) pair as pre-shared key against trojan horse attack strategy is investigated in detail. To prevent this kind of attack strategy, the EPR pairs employed in the quantum private communication is transferred into non-orthogonal entangled states by employing unitary transformations which are actually rotation operations on the quantum signal. Analysis show that the improved scheme is robust against the trojan horse attack strategy without reducing the security against other kinds of attack strategies.