The effects of external electric field (0-0.040 arb. units) on the structure parameter, natural bond orbital charge distribution, electric dipole moment, energy gap and potential energy curve of 5-bromo-2-fluoronitrobenzene (5Br2FNB) molecule were investigated by using DFT/B3LYP/6-311+G(d,p) method in this work. The excitation energy and hole-electron of the molecule under different external electric fields were also studied by TD-DFT/B3LYP/6-311+G(d,p) method. The results show that, with the increase of external electric field intensity, the bond length of C(5)-Br(10) and the electronegativity of Br(10) atom increases, and the HOMO and LUMO energy, energy gap, excitation energy and potential barrier all show an overall decreasing trend, indicating that the stability of the molecule decreases gradually. All the analyses indicate that C(5)-Br(10) bond dissociation has occurred when the external electric field intensity exceeds 0.030 arb. units. According to the hole-electron analysis, it is shown that when the external electric field intensity range is 0-0.010 arb. units, the S₀→S₁ excitation of the molecule is local excitation, and when the external electric field intensity range is 0.020-0.030 arb. units, S₀→S₁ is charge transfer excitation. However, when the external electric field is 0.040 arb. units, S₀→S₁ transforms again into local excitation, which means that the charges of molecule begin to redistribute after the C(5)-Br(10) bond breakage, further confirming that the C(5)-Br(10) bond begins to dissociate when the external electric field intensity reaches 0.030 arb. units. This study has important reference value for deeper understanding of the dissociation mechanism of 5Br2FNB molecule under external electric fields.