In this work, drug-loaded polymer microparticles were prepared by a supercritical solution impregnation (SSI) process with nitrendipine as the model drug and PLLA-PEG-PLLA as the drug carrier. The morphology, size, distribution and functional groups of the drug-loaded microparticles were characterized by scanning electron microscopy (SEM), laser particle size analyzer and fourier transform infrared analysis (FTIR). The effects of pressure, temperature and cosolvent concentration on the drug loading and release property of the microparticles prepared with and without cosolvent were investigated. The drug release kinetics of drug-loaded microparticles was studied with five models. The results indicated that the morphology of the drug-loaded polymer microparticles was not influenced by the SSI process. And the addition of ethanol cosolvent could significantly improve the drug loading of the microparticles. The most satisfied drug loading and the release properties of the microparticles were achieved under 55 °C, 13 MPa and cosolvent ethanol concentration of 3%. The drug could be released for more than 140 h. The analysis of the drug release kinetics showed that the experimental data fitted with Ritger-Peppas model were optimal. According to the release exponent value, the release process of the nitrendipine-loaded microparticles was controlled by Fickian diffusion, which can provides a theoretical basis for drug release of this type of experiment.