Based on the image theory, the oscillation of a cylinder in front of two vertical walls intersecting normally in the otherwise static water is transformed into the oscillating problem of four symmetrically arranged cylinders. The transformed problem is solved by an analytical method based on eigenfunction expansion. Numerical experiment has been performed in order to examine the influence of the distances between the cylinder and the walls and water depth on the added mass and radiation damping of the cylinder. The calculation shows that the added mass and radiation damping of the cylinder in front of the two vertical walls increase abruptly. The added mass and radiation damping oscillate with wave number, and the oscillating frequency increases with the increase of the distances between the cylinder and the walls. Furthermore, the added masses and radiation damping are linear with the water depth in the range of low frequencies.