We perform the finite-difference time-domain (FDTD) simulations and analyze a dual-beam-reflection phenomenon for a Gaussian beam illuminating a Kretschmann configuration composed of a lossless dielectric waveguide between a photonic-crystal-made prism and air. One reflection beam has a small positive shift and the other has a large negative shift in the dual-beam-reflection phenomenon. The FDTD shows that the specific phenomenon takes place just when the corresponding leaky surface mode supported in the Kretschmann configuration is excited. Field profile of the surface mode demonstrates a strong localized stationary field in the dielectric waveguide. We find that the maximum negative lateral shift is -23.23a, corresponding to 4.99 times of the incident wavelength, which is 1.1615 times of the beam waist.