Two improvements to the previous team′s method for measuring the metastable zone width of the flow system at the microscale were proposed.The metastable zone width of the potassium dihydrogen phosphate flow system at the microscale was measured,and the guiding effect of the metastable zone width on the microchannel crystallization process was discussed.The research results showed that the efficiency and accuracy of measurement could be improved by improvements of search method and adjustment of feed temperature.The metastable zone width of potassium dihydrogen phosphate decreases with the increase of flow rate and temperature.When the flow rate was 0.026 m/s,the metastable zone width of saturated potassium di-hydrogen phosphate solution of 40 ℃ and 50 ℃ were 1.8 ℃ and 0.4 ℃,and the metastable zone width became significantly narrower.This finding had important guiding significance for the operation and adjustment of microchannel back-extraction-crystallization technology.In addition,the study showed that BaSO₄ and Prussian blue particles prepared in the microchannel had smaller particle sizes,which was due to the narrower metastable zone width that facilitates the nucleation of crystals.