The deceleration sub-phase during the back squat (BSQ) makes it difficult to stimulate the muscles throughout the full range of motion, and it has only been reported for one load during BSQ. The purpose of this study was to investigate whether a deceleration sub-phase occurs during BSQ with different loads and to assess the influence of load on the deceleration sub-phase duration and negative impulse during the deceleration sub-phase. Sixteen healthy men (mean ± standard deviation: age: 25 ± 3 years; height: 1.73 ± 0.07 m; mass: 83.2 ± 16.1 kg; BSQ one repetition maximum (1RM): 163.8 ± 36.6 kg; BSQ 1RM/body weight: 2.0 ± 0.4) who had performed resistance training for at least 1 year were recruited for this study. The subjects performed parallel BSQ with 0%, 12%, 27%, 42%, 56%, 71%, and 85% of each 1RM on a force plate in a random order. The deceleration sub-phase duration and negative impulse during the deceleration sub-phase were calculated from force-time data. The absolute durations of the deceleration sub-phase were not significantly different according to load except for 27% 1RM and 85% 1RM (p = 0.01). However, as the load increased from 12 to 85% 1RM, the relative duration of the deceleration sub-phase decreased (p < 0.05). The negative impulse during the deceleration sub-phase also increased from 0 to 42% 1RM (p < 0.05). A deceleration sub-phase occurs regardless of the load (0%–85% 1RM), and a large portion of the deceleration sub-phase occupied the concentric phase, with low–moderate loads, and a large amount of negative impulse occurred during the short deceleration sub-phase with a high load.