The impaction well containing an oil-soaked glass fiber (GF) filter of the well impactor ninety-six (WINS) needs to be replaced every five sampling days to avoid particle overloading which affects the cutoff diameter (dpa50) and the sampling accuracy. This study developed a novel modified WINS (M-WINS) which uses water to wash the wetted GF filter substrate clean continuously to eliminate the particle overloading effect and extend the service life of the impactor without the need of impaction well cleaning and replacement of the oil-soaked GF filter. The laboratory test showed that dpa50 of the M-WINS only varied slightly from 2.44 to 2.49 µm while dpa50 of the WINS decreased from 2.44 to 2.05 µm when the loaded particle mass increased from 0–6 mg. The field test showed that dpa50 and the sampling bias of the novel M-WINS met the USEPA requirements (dpa50 = 2.5 ± 0.2 µm and sampling bias <±5%) while the WINS without regular replacement of the well showed the decrease in dpa50 down to 2.12 µm and the increase in the negative sampling bias to −20% after 17 continuous sampling days. Therefore, the current M-WINS can be used as the PM2.5 inlet for unattended, continuous sampling for a long time with a very good sampling accuracy.