The long term trends of precipitation acidity at Guangzhou are analyzed, based on the monitoring data from 1992 to 2012. The results indicate that there was obvious inter annual variation in precipitation pH from 1992 to 2012, the lowest in 1998 with the annual mean pH being 3.43, the highest in 2012 with the annual mean pH being 4.65. The variation of annual precipitation pH values can be divided into two stages: before 1998, precipitation pH decreased fluctuatingly, with an annual mean rate of about -0.14 a-1; after 1999, precipitation pH increased gradually, especially after 2009 increasing steadily, with an annual mean rate about 0.03 a-1. The seasonal distribution of acid rains was uneven, lower in winter and spring and higher in summer and autumn. The annual variation of acid rains was also obvious, the heaviest in February with the average pH being 3.95, the lightest in December with the average 〖JP2〗pH being 4.36. When the sources of air pollution were stable, the greater the wind speed was, the higher the pH value and the lower the〖WTBX〗 K 〖WTBZ〗(precipitation conductivity) was, consequently the smaller the occurrence frequency of acid rains. Wind direction also affects precipitation pH values. The average pH was generally lower when west winds blow in Guangzhou. When precipitation increased, the higher the pH value was, the lower the 〖WTBX〗K〖WTBZ〗.