Herein, we report the introduction of steric hindrance in molecular building blocks to prevent π···π stacking, thus allowing two-dimensional (2D) covalent organic sheets to form three-dimensional (3D) covalent organic frameworks (COFs) through entanglement. Starting from the rationally designed precursors containing a bulky anthracene unit in the vertical direction, a highly crystalline COF (3D-An-COF) was successfully synthesized. Very interestingly, 3D-An-COF was determined as an entangled 2D square net (sql) structure, and the high-resolution data (1.1 Å) obtained by the continuous rotation electron diffraction technique allowed us to directly locate all non-hydrogen atoms. Structurally, the presence of an anthracene group outside the C2h symmetry plane strongly reduces the π···π interactions and promotes the formation of square entanglements. In addition, 3D-An-COF is fluorescent and can be used as a sensor to detect the trace amount of antibiotics in water. This study provides a new strategy for the structural diversification of 3D COFs and will certainly motivate us to construct more entangled COFs for interesting applications in the future.