The stability and transformation of aqueous amino acid at high pressures and temperatures are very important in terms of the formation of interstellar organic matter, delivery of organic compounds of meteorolites, and prebiotic organic synthesis. We studied aqueous alanine saturated solid solution at pressures of several gigapascals and temperatures up to 500°C using an external heating hydrothermal diamond anvil cell (HDAC) through in situ observation and Raman spectroscopy. Five samples of initial pressures of 1.23, 1.98, 2.69, 2.78, and 3.67 GPa, which cause transformation from liquid solution to solid phase, are studied. The transformations of aqueous alanine solution are highly reliant on pressure, and with higher initial pressures, the transitions are retarded to higher temperatures. The melted solid alanine solution of the lowest initial pressure separates out a non-eutectic phase which takes on the characteristic of macro-organic matter, which indicates that amino acids can oligomerize or even polymerize at high pressures and temperatures. As for the highest initial pressure, the solid alanine solution remains almost intact up to the highest temperature of 500°C. The results show that pressure should be required to understand many occurrences of amino acids in space and meteorites.