Nylon 46 with high molecular weight was successfully synthesized in supercritical carbon dioxide (sc-CO2) by one-step three-stage polymerization using the salt prepared from 1,4-diaminobutane and 1,6-adipic acid as a monomer. The polymerization process was divided into three reaction stages: precondensation, postcondensation, and vacuum. To prepare high molecular weight nylon 46, the precondensation of the salts with pH over 7.23 was first carried out at 190 degrees C under 30 MPa for 2 h. Then CO2 in the reactor was exhausted along with removing byproduct water, and the reactor was refilled with dry liquid CO2. Subsequently, the prepolymer formed was subjected to the postcondensation at 280 degrees C for 3 h under 15 MPa. Finally, the reactor was heated in vacuum at 270-280 degrees C for 30-45 min. The highest viscosity-average molecular weight of the nylon 46 sample synthesized from the above process is 48 200 g/mol. The chemical structure and thermal properties of the nylon 46 were characterized by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The nylon 46 sample, as an example, has melting point of 310 degrees C, glass transition temperature of 67 degrees C, starting decomposition temperature of 400 degrees C, and maximum decomposition rate at 450 degrees C.