Abstract: Solubility，super solubility，infrared characteristics，and the influencing factors in cooling crystallization process were systematically studied through the METTLER TOLEDO online crystallization workstation.Results showed that the potas- sium dihydrogen phosphate′s metastable zone was wide in a low temperature region，and was narrow in a high temperature zone，and in 40.1 ℃ it had the widest metastable zone.Potassium dihydrogen phosphate in IR spectra had a good Beer Lambert characteristic.The characteristic peak wavelength was distributed within 2 152 cm-1 and 1 082 cm-1.And the cooling rate was higher，the metastable zone width increased.The stirring rate was higher，the metastable zone width decreased.When pH was 4.0，the crystallization effect was the best，the particle size distribution was the most uniform，and crystal particle size reached maximum.Increasing Fe3+ content，will lead to KH2PO4 metastable zone width increase，and the more the Fe3+ content，the more uneven the size is and the wider the distribution is，so it should strictly control the production process of the Fe3+ mass fraction less than 0.001%. Increasing SO42- content，will narrow the metastable zone，lead to industrial crystallization that is not easy to control，therefore it needs to strictly control the content of SO42- in the production process.

Key words: potassium dihydrogen phosphate, cooling crystallization, METTLER TOLEDO online crystallization workstation