From February 23 to March 2 of 1986, there, was a process of cold front precipitation in Guangdong and Guangxi Provinces.By means of analysing streamline fields of upper air and bottom layer during this pe-riod and of calculating air trajectory for 72 hours, we knew directions and coverages of air stream in different .layers,Having measured acidity and other parameters of precipitation at intervals of time, the results showed: precipitation's acidity became lower gradually when streams of middle and battom layers turned to the south from the north in a passing cold front. We may consider that air streams travelling from the north industry and mining districts, such as Hunan and other remote provinces bring acidulous .compounds far. more than those coming from offshore areas in .the south.