Explosive emission cathodes (EECs) are widely used in the field requiring intense electron beams. The uniform emission of EECs is vital for applications. To effectively evaluate the emission uniformity of annular EECs, this paper proposes a quantitative evaluation model based on the analysis of cathode plasma images and constructs a parameter which is named as the non-uniformity value to describe the extent of emission uniformity. The experiment designed to investigate the cathode emission uniformity under different circumstances demonstrates that the model is valid. The experimental results give quantitative information of uniformity change in explosive emission and show that the non-uniformity values of graphite and aluminum cathodes can be significantly decreased by increasing the guiding magnetic field and the diode voltage.