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Terahertz (THz) focusing metasurface plays a key role in micro-focusing and micro-imaging. Most of the existing focus sets only show the number and deflection of focuses. However, few researches are concerned about the adjustment of focus intensity. Herein we designed a complementary metasurface based on a C-ring and cross-shaped composite slot structures, and the effect of structural thickness on focusing intensity and focal length were studied. We find that the angle of anomalously transmitted beam can be scanned from $17.5^{\circ }$ to $26.5^{\circ }$ when the frequency changes from 1.2 THz to 1.8 THz, while the focal length of the focusing metasurface can be linearly changed from 1000 $\mu \mathrm{\text{m}}$ to 1650 $\mu \mathrm{\text{m}}$. With increase in the thickness of the C-ring area, more interestingly, the focus intensity decreased linearly from 8.0 dB (V/m) to 4.5 dB (V/m). However, the focus intensity showed no obvious change when the thickness of the cross-shaped area was increased from 0.2 $\mu \mathrm{\text{m}}$ to 5 $\mu \mathrm{\text{m}}$. Therefore, our work would enhance the application of THz in high-resolution imaging, new THz device, and flat lens with adjustable focus intensity.