We analyze the molecular mechanism of the chain stretching and orientation for the stress overshoot response of linear and comb polymers under shear using the nonequilibrium molecular dynamics simulation. Different from the strain overshoot behavior of linear polymers, the shear stress peak strain gamma max of comb polymers vs the Rouse-Weissenberg number W i R = tau R gamma displays three scaling law regions, in agreement with previous experimental results [F. Snijkers et al., ACS Macro Lett. 2, 601-604 (2013)]. In contrast to experiments, our simulations visually reveal the stretching and orientation dynamics of chain segments at the stress overshoot. According to the detailed information of stretching and orientation of the segment with different lengths, it is found that the size of a tension blob decreases with increasing W i R, and it follows a power-law with an exponent of -0.6 for a linear system. With respect to the comb systems, the size of a tension blob decreases accordingly with grafting density under the same shear rate. Unambiguous molecular pictures at the stress overshoot are also given at different shear rates for linear and comb polymers.