We experimentally study the light coupling behaviors between the transmittinglight and reflected light in ycut lithium niobate crystals. When the epolarized incident light is launched into the crystals along +y axis and -y axis, respectively, we find that the time behaviors of the reflectivity and transmissivity are almost identical for both cases, respectively. Whereas when the opolarized incident light is launched into the crystals along +y axis and -y axis, respectively, the time behaviors of the reflectivity and transmissivity for both cases are obviously different. We investigate the physics mechanism of these phenomena preliminarily.