The appearance of haze has importance indication meaning to air quality, while the fog or light fog has specific weather indication relating to particular weather system. With the rapid development of economic scale and urbanization, the phenomena of haze or haze weather have become increasingly serious. The standard criterion by using relative humidity to distinguish haze or fog in many northeast coastal provinces is generally on the low side, leading to much haze weather recorded as light fog or fog. In the near surface atmosphere there always exists haze, while fog drop hardly exists. Only under the enough supersaturation which is higher than that the maximum of supersaturation can the haze nuclei become fog drop through moisture absorption growing, such a process is not easy to happen. Under non-saturation conditions, not only the insoluble haze nuclei are impossible to transform into fog drop, but also the soluble ones are commonly impossible. The observational data show that the extreme relative humidity is 91% when fog occurred, there is no fog when the relative humidity is lower than 90%. The most important physical process to form fog drop is by falling the temperature to the saturation point. It is hardly impossible to form fog drop through the natural moisture absorption growing of haze nuclei. The distinguishing of fog or haze should be made according to the change of weather system combining with the macro-characteristics. It is rational by using the threshold value of relative humidity 90% as an assistant criterion to distinguish fog or haze.