Eight years of data on haze and visibility (2003–2010) and one year of (2010) data on surface meteorological elements (relative humidity, wind speed, air temperature), visibility and the concentrations of air pollutants (PM2.5, SO2, NO2 and O3) measured each hour of each day were analyzed using correlation analysis to investigate the main factors influencing haze in Hangzhou, China. The occurrence of hazy weather has become more frequent over the past eight years in Hangzhou, and haze appears about 160 days per year. The occurrence of haze during the day was more frequent in the spring and the winter and less frequent in the summer and the autumn. Low visibility occurred in the morning, and the maximum visibility occurred in the afternoon period. The results of the statistical analysis show that the relative humidity and the concentration of PM2.5 played the most important roles in reducing visibility. The correlation coefficients between the concentration of PM2.5 and the concentrations of O3, SO2 and NO2 indicate that O3 and NO2 are the dominant factors contributing to PM2.5 pollution, which, in turn, can lead to haze. To reduce the number of haze days, greater concern and more countermeasures should be taken to decrease the O3 and NO2 pollution in Hangzhou, China.