A microscopic observation system with a temperature-pressure-controllable sample cell and methods. The system can be configured to perform common optical microscopic observation and polarizing microscopic observation. The system is composed of a visual autoclave, a temperature control component, a rapid cooling component, a pressure control component and an optical imaging system, and can be configured to observe an evolution process of microstructures of polymer materials in specific atmosphere and rapid temperature and pressure changing environments in a scale of 1 μm-1 cm. A novel characterization means for researching a condensed state evolution law of polymers in high-pressure environments, and also a new thought for deep reveal of the polymer crystallization mechanism and regulation of crystallization and phase separation behaviors of the polymer materials.