It is significant to objectively measure the skin erythema in clinical dermatological diagnosis and new dermatological drug tests. The current measurement methods are biased by the melanin in the epidermis and the incidence light intensity. Based on the skin optics model,we propose a new parameter which is defined by the logarithm difference of the inverse of diffuse reflection light between normal skin and erythema to measure the change of erythema. The theoretical analysis shows that this new parameter can reflect directly the deviation of the concentration of haemoglobin from the normal range under the assumption of keeping the melanin concentration constant and the illuminating light uniform, The experimental study shows that the new method is hardly influenced by the illuminating light intensity,and can be effectively utilized in clinical diagnosis.