ABSTRACT: This work presented a mechanism study on the thermal stratification of vertical cylindrical heat storage tank in insulation process. The development of the temperature and flow fields with the influence of the thermal insulation layer was specially studied. Every wall was considered as the third boundary condition and the flow was laminar. The experimental and numerical methods were both carried on this study. The numerical method was validated by comparing the experimental and numerical results. The results of this work indicate that: the insulation process of the cylindrical tank can be divided into non-steady state and quasi-steady state. In non-steady state, the flow and thermal change in fluid regime is mainly controlled by side wall and top wall, and the temperature in insulation layer increases gradually from inside to outside at first and then rises overall. In quasi-steady state, two regimes which are called stratification regime and thermal fluid regime are formed in the fluid region and the temperature filed in the insulation layer is relatively steady and drops integrally with time. The highest temperature on the outer wall of the thermal insulation layer can be regarded as a sign of the division of the non-steady state and the quasi-steady state.