The Galerkin method is used to solve the diffusion equation of the distribution function for rigid rod-like polymers and the pseudo-spectral method is used to solve the momentum equation that includes the stress tensor determined by distribution function of polymers. The properties of the coherent structure in a two-dimensional time-developing viscoelastic mixing layer are given and the most probable orientations of the multibead rods as well as the stresses of polymers are obtained. The results indicate that adding polymers to the Newtonian fluid will result in a stronger vorticity diffusion, accompanied by weaker fundamental and subharmonic perturbations, less intense vortex roll-up of the mixing layer, lower oscillation frequency of fundamental energy, slower rotational motion of neighboring vortices and shorter life times of eddies. The results also show that the stretching of large eddies will have a great effect on the behavior of rod-like molecules, since the most probable orientations of rods seem to coincide with the local fluid stretching directions.