During the operation stage, parallel bridges may become nonparallel as a result of unequal load distribution between two parallel bridges and other special conditions. Aerodynamic performance could change significantly under nonparallel positions and become different from that under parallel positions. In this paper, the stationary aerodynamic performance of two parallel bridges under various nonparallel positions during operation stage is studied through a series of wind tunnel tests. This includes the investigation of two horizontal gap distances (HGDs), five relative vertical displacements (RVD) and five relative torsional displacements (RTD). First, sectional models of two closed box girders were tested in smooth flow for stationary aerodynamic force coefficients. An optimum iteration method was then used to calculate the structural displacements and torsional divergence critical wind velocities (U-cr) of two assumed suspension bridges under stationary aerodynamic force. The research outcomes demonstrated that the changes of stationary aerodynamic force coefficients are dependent on the relative displacements of two girders and wind attack angles. In addition, it was revealed that interference effects are detrimental to stationary aerodynamic instability of two bridges with a larger gap-width ratio (i.e. D/B = 1), which is related to the aerodynamic shape of girders and bridge structures. Further, the Ucr of the leeward bridge significantly decline when the vertical position of the leeward bridge become higher that of the windward bridge. Most importantly, it showed that the combination of RVD and RTD (e.g. RVD = -20mm and RTD = -2 degrees) could potentially lead to the worst stationary aerodynamic performance by decreasing U-cr of the windward and leeward bridge with 12.03% and 7.89%, respectively.