Depletion of atmospheric ozone layers is more and more serious. Alkyl halides dissociate under the solar UV radiation with the product of free halogen atoms, which greatly damages the ozone layer and is the main culprit for the depletion of ozone layers. In this paper, methyl iodide is chosen as a calibration system of velocity map imaging. Velocity map images of iodine atom I (2P3/2) at different focus voltages are obtained in the dissociation of methyl iodine under an UV radiation of ～266 nm by techniques of velocity map imaging and REMPI (Resonance Enhanced Multiphoton Ionization). The magnification factor N of velocity map imaging system is measured to be 1.13. Photodissociation dynamics of 1, 4-C4H8BrCl under an UV radiation of ～234 nm is investigated on this velocity map imaging system. The speed and angular distributions of the fragments Br(2P3/2) and Br* (2P1/2) atoms in the dissociation are obtained and analyzed. Experimental results suggest that the dissociation of 1, 4-C4H8BrCl to both Br(2P3/2) and Br* (2P1/2) atoms under an UV radiation of ～234 nm happens promptly along the C-Br bond via repulsive surfaces after excitation. The anisotropy coefficient values are obtained from angular distributions of imaging of the fragments Br (2P3/2) and Br* (2P1/2) atoms, by which the ratio between perpendicular transition and parallel transition for those two dissociation channels are calculated. In addition, photodissociation mechanisms of CH2BrCl, 1, 2-C2H4BrCl, 1, 3-C3H6BrCl and 1, 4-C4H8BrCl at an UV radiation of ～234 nm are compared, and the dependences of dissociation mechanisms of dihalogen alkyl compounds on size of the alkyl radical are obtained.