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Based on fractal theory, a regular fractal is used to construct symmetrical reef models (e.g., cube and triangle reef models) with different fractal levels (<i>n</i> = 1, 2, 3). Using the concept of fractal dimension, we can better understand the spatial effectiveness of artificial reefs. The void space complexity index is defined to quantify the complexity of the internal spatial distribution of artificial reefs models under different levels. The computational fluid dynamics (CFD) flow simulation approach was used to investigate the effects of void space complexity on the flow field performances of the symmetrical artificial reef models. The upwelling convection index (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>H</mi><mrow><mi>u</mi><mi>p</mi><mi>w</mi><mi>e</mi><mi>l</mi><mi>l</mi><mi>i</mi><mi>n</mi><mi>g</mi></mrow></msub><mo>/</mo><msub><mi>H</mi><mrow><mi>A</mi><mi>R</mi></mrow></msub></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>V</mi><mrow><mi>u</mi><mi>p</mi><mi>w</mi><mi>e</mi><mi>l</mi><mi>l</mi><mi>i</mi><mi>n</mi><mi>g</mi></mrow></msub><mo>/</mo><msub><mi>V</mi><mrow><mi>A</mi><mi>R</mi></mrow></msub></mrow></semantics></math></inline-formula>), wake recirculating index (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>L</mi><mrow><mi>w</mi><mi>a</mi><mi>k</mi><mi>e</mi></mrow></msub><mo>/</mo><msub><mi>L</mi><mrow><mi>A</mi><mi>R</mi></mrow></msub></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>V</mi><mrow><mi>w</mi><mi>a</mi><mi>k</mi><mi>e</mi></mrow></msub><mo>/</mo><msub><mi>V</mi><mrow><mi>A</mi><mi>R</mi></mrow></msub></mrow></semantics></math></inline-formula>) and non-dimensionalized velocity ratio range were used to evaluate the efficiency of the flow field effect inside or around artificial reefs. The surface area and spatial complexity index of artificial reefs increase with increasing fractal level. The numerical simulation data shows that the Menger-type artificial reef models with a higher spatial complexity index have better flow field performances in the upwelling and wake regions. Compared to the traditional artificial reef models, the upwelling convection index (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>V</mi><mrow><mi>u</mi><mi>p</mi><mi>w</mi><mi>e</mi><mi>l</mi><mi>l</mi><mi>i</mi><mi>n</mi><mi>g</mi></mrow></msub><mo>/</mo><msub><mi>V</mi><mrow><mi>A</mi><mi>R</mi></mrow></msub></mrow></semantics></math></inline-formula>) and recirculating index (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>V</mi><mrow><mi>w</mi><mi>a</mi><mi>k</mi><mi>e</mi></mrow></msub><mo>/</mo><msub><mi>V</mi><mrow><mi>A</mi><mi>R</mi></mrow></msub></mrow></semantics></math></inline-formula>) of <i>n</i> = 3 fractal cube artificial reef increase by 37.5% and 46.8%, respectively. The efficiency indices of the upwelling region and wake region around the fractal triangle artificial reef model are 2–3 times those of the fractal cube artificial reef model when the fractal level is 3.