Objective:,2,To optimize the preparation process of thermosensitive 20(S) -Protopanaxadiol(PPD) microemulsion-gel composite system, and investigate its gelation temperature, gelation time, appearance characteristics, rheology, and drug release ,in vitro, and mechanism.,Methods:,2,Based on the preparation process of PPD microemulsion in the previous stage, the thermosensitive PPD microemulsion-gel composite system was prepared by cold-dissolving method.The influence of different excipients on the gelation temperature was investigated by single factor test to determine the basic formula, including poloxamer 407(P407), hydroxypropyl methyl cellulose(HPMC), sodium alginate(SA) and glycerin.Then, the optimal formulation and process of the composite system were selected according to the gelation temperature, gelation time and ,in vitro, release.The morphology was observed by scanning electron microscope(SEM) and transmission electron microscope(TEM) .The rheological properties were detected by rotary rheometer.The drug release rate ,in vitro, was determined by dialysis method, and the equations were fitted to explore the drug release mechanism.,Results:,2,The optimized formulation of thermosensitive PPD microemulsion-gel composite system was as follows: 30% P407 aqueous solution, 0.6% HPMC, 0.2% SA, 4% glycerol, 0.05% Nipagin ethyl ester, and the volume ratio of P407 aqueous solution to PPD microemulsion was 2∶1.For thermosensitive PPD microemulsion-gel prepared under this condition, the gelation temperature was(30.9±0.1) ℃and the gelation time was(146.7±4.4) s.SEM showed porous structure and loose pores, and TEM showed that the microemulsion particles were round and without adhesion.The preparation showed good thermosensitivity and good fluidity at room temperature.It could undergo phase transition from solution to gel when slight below skin temperature.The release time ,in vitro, was 72 hours.The drug release process ,in vitro, conformed to Weibull equation, which was the synergistic effect of drug diffusion and skeleton dissolution.,Conclusion:,2,This thermosensitive PPD microemulsion-gel composite system shows simple preparation process, suitable gelationtemperature and time, andexhibits thermosensitivity and sustained release, which can meet the local medication needs of the skin.