Surface charge accumulation and decay behaviors of dielectric materials are the key factors restricting the development of high voltage direct current power equipment. For flat samples, the density of surface charges deposited by corona can be regarded as a linear change with the surface potential. For this reason, the behavior of surface charge decay can be directly related to that of surface potential. According to the corona charging process, the surface charge deposition and detrapping process, as well as the charge transport process in the bulk, we may establish a physical model dynamic response to the surface potential. Influences of grid voltage, relative permittivity, and bulk conductivity on the surface potential decay process can be obtained through calculating the surface potential decay behaviors of epoxy resin. The higher the grid voltage, the faster the surface potential decays. At the typical parameter value of epoxy resin (relative permittivity 3.93, bulk conductivity 10-14 S m-1), the normalized decay rate can be fitted by two straight lines in a log-log plot; moreover, the calculated results show a linear variation of power factors with the grid voltage, while the power function shows a relationship between the characteristic time and the grid voltage. The bigger the relative permittivity, the slower the surface potential decays. In the typical parameter area of epoxy resin (relative permittivity 3-4), the surface potential decay time constant increases from 1720 s to 2540 s, showing a linear variation. Also the bigger the bulk conductivity, the faster the surface potential decays. In the typical parameter area of epoxy resin (bulk conductivity 10-15-10-13 S m-1), the surface potential decay time constant decreases from 24760 s to 260 s, showing a power function relationship.