A 2D axisymmetrical numerical model was established to investigate the drag coefficient during the natural deceleration of an underwater projectile with supercavity at a relative low cavitation number. Based on dynamic mesh technology, the variations of unsteady drag coefficient with time have been studied under several model slenderness ratios at a range of initial cavitation number. The simulation results show that the variations of the supercavity length lag behind that of the supercavity thickness during the same supercavitation movements. Whatever the model slenderness ratio is, the model drag coefficients are quasi-linear ascent over time. The cavitation number is ascent over the deceleration, when the cavitation number is low, the curve slope of the drag coefficients is small and the drag coefficient is almost unchanged and when the cavitation number is high, the curve of the drag coefficient is quick ascent. When the initial cavitation number is low enough, the changes of the drag coefficient with the initial cavitation number are very small and the variation of the effect of supercavitation drag reduction with the descent of the initial cavitation number is no longer obvious.