Abstract: Atmospheric visibility（horizontal visibility，referred to as visibility）can indicate the regional atmospheric environmental quality to a certain extent. According to the monitoring data from the State-Controlled Air Automatic Monitoring Station at Xianning from 2017 to 2018，the relationship between visibility and PM_2.5（mass concentration，the same below）was analyzed by the curve-fitting method in different relative humidity（RH）. The results show that the visibility and PM_2.5 concentration in Xianning have obvious seasonal variation characteristics. In summer，PM_2.5 concentration was lower and visibility was higher，and in winter，PM_2.5 concentration was higher and visibility was lower. Using SPSS17.0 software to perform curve fitting， it is found that the visibility and PM_2.5 concentration are related by a power function. When 50% < RH ≤ 95%，the fit is good，and when RH is 65%，the critical concentration of PM_2.5 is 51 μg/m³ when haze is observed in Xianning. The least-squares method is used to obtain the fitting equation of visibility about PM_2.5 concentration and relative humidity. It has been verified that the simulation effect is good，and it can be used to predict the visibility and PM_2.5 concentration.

Key words: visibility, PM_2.5, characteristics of change, relative humidity（RH）, fitting curve