Abstract: It is the first time that concentration of organic acid (formic acid and acetic acid in precipitation for Lanzhou City is continuously determined and studied by IC method. The results show that organic acid accounts for 6.88 percent of negation ion and 2.50 percent of total ion in precipitation for the region.The ion concentration range 0.31-1.63 (mg/L) for formic acid and 0.50-0.91 (mg/L) for acetic acid. The balance condition between negative ion and positive ion has been improved after measuring orga nic acid.