In general, disordered speckles occur when a laser beam passes through a scattering medium. Based on wavefront phase modulation technology, we apply a continuous sequential algorithm combined with a four-step phase-shifting technology to phase modulation of the wavefront of the incident laser so that it can focus on the target position when the laser passes through the scattering medium. The experiments primarily investigate the focusing of the laser passing through the strong scattering-medium samples with different thicknesses, and discuss the influence of the scattering-medium thickness upon the enhancement factor of intensity at the focal point. The experimental results show that with the increase in the thickness of the scattering medium, the size of the focal spot and the enhancement factor of intensity of the focal point at the target position decrease.