The running up of water onto beaches is of great significance in coastal engineering. Theoretical and experimental studies in this field can be found in [1]–[7]. Recently, some numerical methods associated with this problem have appeared [4]–[7]. When the wave climbs up the inclined sea shore, there are moving boundaries which are unknown in advance. Under the potential flow assumption, Kim [5] calculated the climbing of a soliton on an inclined beach using BEM and compared his results with experiments. The results are good only if the wave height is relatively low with a slope angle of less than 15°. When height/depth = 0.2, a disagreement is evident. In this paper, a water film is imagined to cover the whole beach and a definite boundary is settled at this film. This water film is considered part of the water body. So, the moving boundary problem is changed into a fixed boundary problem and the original boundary line becomes a break line of the water surface. However, the calculation with such a break line would be unstable with ordinary finite difference methods. So, a specially designed filter is used to smooth the computation. The calculated results are consistent with experiments and some interesting phenomena have been discovered.