In order to simulate the response of hexanitrostibene (HNS) pellets in exploding foil initiators under heat environment,sealed sample tubes were designed. The cook-off tests of HNS pellets in Φ3 mm×4 mm,Φ4 mm×5 mm and Φ4 mm×6 mm with TMD in charge density of 90%, were carried out at the heating rates of 3.3 ℃·h^-1, 5, 10 ℃·min^-1. The reaction responses were determined by the damage or distortion of the sample tubes,the damage of its vulnerable place and the depth of steel dents. The test results were compared with those of unsealed pellets. The results indicate that the responses of HNS pellets under sealed condition are severe and the initial reaction temperatures of sealed HNS pellets （302.5-314.7 ℃） are lower than that of unsealed ones（328-335 ℃）. Under the heating rate of 5 ℃·min^-1 or 10 ℃·min^-1, the initial reaction temperature decreases as the charge weight increases. The bigger charge structure accumulates heat more easily, and it′s more sensitive to heat.