To address the challenge of decreased measurement accuracy and repeatability caused by uneven temperature distribution in the outer bucket of traditional calorimeters, an outer-bucket spiral isothermal system that integrates a spiral guide tube with a semiconductor temperature control module were proposed and constructd. To verify its performance, systematic experiments from three perspectives: temperature uniformity, all-weather stability, and single-process stability were conducted. The results show that within the target temperature range of 15~30 ℃, the maximum axial temperature difference in the outer bucket ranges from 0.014 3 to 0.031 0 ℃; in all-weather tests with ambient temperature fluctuations exceeding 10 ℃, the temperature fluctuations at all points in the outer bucket are less than 0.05 ℃; and in single complete heat generation tests, the temperature fluctuation range in the outer bucket is less than 0.01 ℃. It was confirmed that the spiral isothermal system effectively eliminates the thermal stratification seen in traditional outer buckets, creating a highly stable isothermal environment for the inner bucket, significantly enhancing the calorimeter's environmental adaptability and measurement reliability, and providing an effective technical solution for high-precision calorimetry.