Based on the data of dew and frost from 2009 to 2010 recorded by automatic weather station(AWS), manual observation data in Miyun and Dianbai National Reference Climatological Stations and the AWS records and the weather observation data of Pinggu Ordinary Climatological Station from December 2010 to May 2011, the relationships between surface meteorological factors and dew or frost were analyzed. The results show that: the grass temperature and the difference between grass temperature and dew point temperature can be an indicator of dew and frost; 95.0% dews occur in the conditions of the difference being <1.1℃ in Dianbai Station, and 95.0% dews and 95.0% frosts occur when the difference is <1.2℃ and <1.3℃ respectively in Miyun Station. Dew mainly occurs when the minimum grass temperature is -0.5℃, from the analysis of automatic present weather observation data in Pinggu Station, it can be seen that dew occurs when the temperature is above zero, while frost occurs when the temperature is below zero. In many cases, the water vapor content of atmosphere at the height of thermometer screen is not saturated, while the water vapor content of groud hugging (or close to the surface features) atmosphere is saturated. In the conditions of the difference between the minimum grass temperature and the dew point temperature in a day being <3.1℃, the proportion of no dew and frost is 16.8% and 11.7% in the two stations. The proportion of positive difference between grass temperature and dew point temperature in the South China is larger than that in the North China because the soil moisture in the South China is larger than that in the North China.