In order to improve the solubility of polyaniline, the chloromethylation of polyaniline by using paraformalclehyde as chloromethylation reagent was firstly reported in this paper. The chlorine methyl group was succesfully introduced into the chain structure of polyaniline which was confirmed by electron energy spectrum analysis. Influences of reaction parameters, such as temperature, time, and amount of paraformaldehyde was investigated by orthogonal experiment. Experimental results showed that the amount of paraformaldehyde is the biggest influence factor on the reaction, and the influence of temperature is the minimal one. Under the optimum conditions, the obtained modified polyaniline contain about 3 wt% of chlorine content under the conditions of temperature was 60 oC, amount of paraformaldehyde is 20 g and the reaction continued 8 h.