Electronic diode plays an important role in electronic circuits owing to its capability of unidirectional movement of the current flux. An optical diode offers unidirectional propagation of light beams, which plays key roles in the all-optical integrated circuits. Unidirectional wave propagation requires either time-reversal or spatial inversion symmetry breaking. The former can be achieved with the help of nonlinear materials, magnetic-optical materials and so on. The realization of these schemes all needs the external conditions (electric field, magnetic field or light field), and thus their applications are limited. In contrast, spatial inversion symmetry breaking can make up for this shortcoming and has been widely studied. Through breaking the structure's spatial inversion symmetry, much research demonstrated that the unidirectional light propagation could be achieved in a photonic crystal structure. Specially, the optical diode based on the photonic crystal heterojunction has been drawing much attention. Though relevant studies have been reported, how to find a more simple structure to realize high-efficiency optical diodes is always pursued by people. The previous design of optical diode is generally based on the orthogonal or non-orthogonal photonic crystal heterojunctions. However, the comparative analysis of the two types of heterojunctions has not been systematically carried out. The transmission characteristics of two-dimensional orthogonal and non-orthogonal silicon photonic crystal heterojunctions are obtained and compared. Firstly, the directional band gap mismatch of two-dimensional square-lattice silicon photonic crystals with the same lattice constant but different air hole radii is calculated by the plane wave expansion method. The band structure indicates that in a certain frequency range, one photonic crystal is the omni-directional pass band, while the other has directional band gap. This is just the necessary condition for the unidirectional light transmission through the photonic crystal heterojunctions. Therefore, the heterojunction constructed by the two photonic crystals is expected to achieve optical diode. Based on this, the orthogonal and the non-orthogonal heterojunctions are proposed. Their transmission spectra and field distributions are calculated by the finite-difference time-domain method. The results show that the unidirectional light transmission can be realized by the non-orthogonal heterojunction structure (unidirectional transmission efficiency reaches 45%) but not the orthogonal heterojunction structure. That is to say, the realization of unidirectional transmission is closely related to the orientation of the hetero-interface. Moreover, the non-orthogonal photonic crystal hetero-interface is optimized. It is found that the unidirectional transmission efficiency increases to 54% and the overall increases by 10%. More importantly, it greatly improves the performance of optical diode for its simple structure and small size, and provides another more effective design method.