The bandwidth limitation of high-power light sources and high-sensitivity detectors, as well as the signal inter-symbol interference caused by the multipath effect of seawater channels, seriously affect the performance of underwater wireless optical communication (UWOC) systems. Aiming at this problem, an inverse hyperbolic sinusoidal function variable step size dual-mode blind equalization algorithm based on fractional spaced equalizer is proposed in this paper. The algorithm combines the variable step size constant-to-mode fractional spaced equalizer algorithm and the decision directed least mean square mode-fractional spaced equalizer algorithm through the mixed parameters for convex combination, and iteratively updates the mixed parameters to achieve the adaptive switching between the two equalization algorithms does not need to manually set the switching threshold, and has a small mean square error under the condition of ensuring rapid convergence, and at the same time, it does not need to occupy additional bandwidth. The simulation results show that the dual-mode blind equalization algorithm can quickly converge under various signal-to-noise ratios, reduce the steady-state mean square error, effectively suppress inter-symbol interference, and improve UWOC systems error performance.