The relationship between quantum dot and energy level was analyzed using a simple model and the equation about the energy level and quantum dot size was developed. The electron wave function within quantum dot does not fully meet the standing wave condition for the existence of surface wave attenuation, so a correction factor expression was introduced combined with the equation about energy level of quantum dot. And the calculation method of the correction factor was introduced. The theory calculation shows that the gap between energy level of quantum dot is not only related to the quantum dot size but its boundary conditions. The surface potential barrier has an important impact on the energy level of the quantum dot and the energy gap especially for non-smooth surface.