Abstract: Field residue decline study of metalaxyl-M in potato and soil were studied. The GAP of metalaxyl-M in potato application was discussed and recommended. Results of field trial showed that the half-lives of metalaxyl-M in potato plants and soil were 1.0~3.7 days and 13.6~14.4 days, respectively, at double of the recommended dosage(144 g/hm2). The 14th day results revealed that the metalaxyl-M residues in potato were below 0.02 mg/kg after the potato was sprayed by 3~4 continous applications at the dosages of active ingredients 72 g/hm2 and 144 g/hm2. It's recommended that the pre-harvest interval (PHI) of metalaxyl-M in potato set at 14 days.