When a water jet freely falls onto a rigid plane with a low flow rate, light rings around the water jet can be observed if the point that the water jet contacts with the plane is illuminated by a laser beam. It is proved experimentally that stable ripple structures surrounding the contact point are essential to form the light rings of water jet. The equation of corrugated stability can be established based on the Bernoulli′s equation and the dispersion relationship of disturbance wave propagation derived by the Plateau-Rayleigh instability theory, and then experiments are designed to investigate the properties of the light rings of water jet. The theoretical and experimental results show that the spacing, brightness and quantity of the light rings are straightly related to the falling height and the initial flow velocity of the water jet, the variety of the rigid plane and the depression angle. The spacing between the light rings becomes smaller with the increasing falling height and initial flow velocity of the water jet. The less the energy loss of laser in a rigid plane is, the brighter the light rings are. There is a proper depression angle range to observe the light rings. It is concluded that the bigger the depression angle is, the more the observed light rings are. Consequently it is better to overlook the light rings for a satisfied observation effect.