Abstract: Thepopular theory for the instability of longshore currents is thelinear shear instability theory. It is assumed that theinstability mode with the largest growth rate dominates theinstability of longshore currents. However, the assumption cannotbe used to some case from the experimental results. In the presentpaper, milti-mode unstable process (or behavior) has been analyzedwhich is in good agreement with our previous experiment results.Velocity profile has great effects on the growth mode of theinstability of longshore currents. Specially, variations can leadto multi-mode of the instability of longshore currents, viz. morethan two peaks occur in growth rate curve. In present paper, weanalyze the effect of frontshear and backshear simultaneousvariations and individually of the both on the multi-mode of theinstability of longshore currents using analytic velocityprofiles. The analysis shows that the wave development(orphenomena, formation，evolution) in our experiment can beattributed to the frontshear instability.