The slow cook-off and fast cook-off tests of hydroxyl terminated polyether（HTPE） propellant have been carried out as a function of sizes of samples (3 types). The temperature distribution inside the samples during the tests has been analyzed by Fluent software. The results show that during the slow cook-of tests, there are few differences in the response temperature of the samples. If the response degree of slow cook-off tests increases significantly, the corresponding fast cook-off test increases slightly. The response levels for slow cook-off tests of HTPE propellants at small-scale, medium-scale and large-scale sample are combustion, explosion and detonation, respectively. In comparsion, the corresponding response levels from the fast cook-off tests are combustion, combustion and deflagration, respectively. In the slow cook-off simulation, the ignition points of the three different size samples are all the same at the center of the propellants. In the fast cook-off simulation, the ignition point locations of small and medium sample are in the annular region of the angle between the propellant and cap, whereas there are multiple ignition points in the large sample located in the middle of the case. In the experimental study of the cook-off characteristics of solid rocket motor, the effect of the grain structure and sample size on heat transfer mechanism must be fully considered in the rational design of small-scale analog motor.