In order to meet the needs of verification and calibration of high-precision temperature measuring instruments, a high-precision thermostatic bath was designed. In terms of structural design, the inner cylinder structure of the thermostatic bath was optimized. The inner cylinder adopted an upper stirring structure, and the stirring chamber and the working chamber were separated. The upper and lower connecting structures were connected as a whole, and the rectifying grid and the working medium circulation system were designed to ensure the stable circulation and heat exchange of the working medium in the cylinder, and improve the uniformity and stability of the temperature field distribution. In the temperature control algorithm, the incremental proportion integration differentiation (PID) algorithm was used, the PID coefficients were self-tuning by the particle swarm algorithm, and the closed-loop negative feedback PID structure was used to achieve precise temperature control of the constant temperature bath. The selfdeveloped software was used to test the performance of the constant temperature bath. The test results showed that the designed constant temperature bath can realize a rapid cooling and heating process, and the temperature field stability at -10℃ was 0.0011℃/min, and the maximum temperature difference at the upper level in the working cavity was only 0.0034℃, the maximum temperature difference in the lower horizontal plane was 0.0020℃, and the maximum temperature difference in the inner cylinder working chamber was 0.0034℃. The entire system had a high degree of automation, stable temperature control, and can meet the national performance requirements.