We investigated the mechanism of erosion on Bijia Mountain tombolo, a famous gravel tombolo in China. Both numerical simulation and experiment were carried out. First, the mechanism was deduced by reviewing changes in cross section, crest elevation, gravel composition, and overall shape in the tombolo between 1985 and 2005 and changes in wave condition near the tombolo. Then, an overall physical model test was carried out to verify this mechanism. Based on the experimental data, we conclude that the breaking of original balanced wave conditions is the main reason for the erosion. When the bidirectional alternating wave action changed into a unidirectional wave field, the original equilibrium was broken. Strong easterly waves (over 25 and 50 year return periods) eroded the tombolo, while wave effects from the ESE and SE directions at mean water level were the main causes of the tombolo’s destruction, followed by wave effects from the E direction. The main effects of the waves decreased the height and skewed the cross section. The strong erosion happened at mean water level, where the waves broke near the top and induced stronger current. The positions of the troughs had a good relationship to the wave-energy distribution. Further study was performed by cross-sectional model test to check out the effects of current on the deep trough along the tombolo. The tombolo was severely eroded due to the joint effects of eastward waves and tidal currents. This investigation on the mechanism of the erosion of Bijia Mountain tombolo can be used in further studies of the restoration of the tombolo.