To accurately evaluate the effect of DC voltage on oil-water interfacial tension, this study conducted a systematic oil-water electrolysis experiment to explore the changes and mechanisms of oil-water interfacial tension under different voltage conditions. The experiment selected heavy oil and CaCl2 water type, and applied DC voltages of 0V, 5V, 10V, and 15V for treatment, respectively. By measuring the pH value and ion content of the aqueous phase, as well as analyzing the composition and functional group content of the oil phase, the potential principle of reducing oil-water interfacial tension by direct current voltage was explored in depth. The results showed that with the increase of DC voltage, the interfacial tension between oil and water significantly decreased, with a maximum reduction of 42.66%. The interfacial tension between oil and water at different voltages is 31.93%, 27.05%, 21.89%, and 18.31%, respectively. In addition, the solubility of Ca2+^{in CaCl}2 aqueous form decreases with increasing temperature, from 2.81 mg/L to 2.25 mg/L, which weakens the influence of DC voltage on the oil-water interface. However, the Na+ content increased from 0.50 mg/L to 0.72 mg/L, which is beneficial for further reducing interfacial tension. Under the influence of electrochemical effects, the pH value of the aqueous solution increased from 7.32 to 10.96, and alkaline conditions promoted the reaction of acidic substances in crude oil, generating surface active substances such as carboxylic acid salts. Through Fourier transform infrared spectroscopy analysis, the normalized content of carboxylate was 1.40, 1.51, 1.90, and 4.11 at different voltages, indicating that the increase in carboxylate content further promoted the decrease in interfacial tension. In addition, electrochemical reactions promote the simplification of crude oil composition. The experiment showed that the content of asphalt and resin was 18.63% and 12.96%, 14.62% and 12.02%, 13.26% and 10.25%, 12.11% and 10.06%, respectively, under different voltages, while the content of saturated hydrocarbons was 47.57%, 51.00%, 54.81% and 55.89%, respectively. This study enhances the understanding of the mechanism by which direct current electric fields reduce oil-water interfacial tension, providing theoretical support for promoting the application of direct current electric fields in improving crude oil recovery. Through in-depth exploration of the formation mechanisms of electrochemical effects and interfacial active substances, valuable references have been provided for the optimization of direct current electric field technology in practical oilfield applications.