The polarization effect in gallium nitride (GaN) will reduce the efficiency of light-emitting diodes (LEDs), so the research on non-polar and semi-polar bulk GaN has received extensive attention. The purpose of our research is to optimize the growth of bulk GaN by exploring the luminescence characteristics between different polar directions of GaN and the fundamental mechanism of impurity doping. In this paper, the bulk GaN single crystals with different polar directions grown laterally by the Na-flux method are used as the research object. The optical properties and impurity distribution characteristics of bulk GaN with different polar directions were compared, and the origin of yellow luminescence (YL) band of bulk GaN and its influencing factors were discussed. At the beginning, the optical properties of GaN bulk single crystals with different polar directions by liquid phase epitaxy (LPE) were studied by cathodoluminescence (CL) and photoluminescence (PL). The experimental results show that optical properties of bulk GaN with different growth polar directions are different. The CL and PL characteristics of ［1122］ and ［1120］ GaN grown in the lateral direction are similar, but the spectra of ［0001］ GaN is quite different. The impurity peak of the photoluminescence contains two shoulder peaks, peak 1 (2.2 eV) and peak 2 (2.6 eV), which accounted for different proportions in different polar directions. We speculate that they are related to the CNON complex and the 0/ + energy of CN defect respectively. Then through time of flight secondary ion mass spectrometer (TOF-SIMS) element analysis, the distribution of C impurity is relatively uniform. There is a big difference in the distribution of oxygen impurity. In the ［1122］ GaN region, the oxygen impurity content gradually increases along the growth direction. Combined with the increase in the intensity of the peak 1 at 2.2 eV in the PL spectra, this phenomenon further proves that there is a positive correlation between the intensity of the 2.2 eV peak and the oxygen impurity.