<p>This study proposes a novel cell-based smoothed 3-node triangular flat shell element, including real drilling degrees of freedom for the analysis of porous plates and shells, based on higher-order shear deformation theory (HSDT). This theory eliminates the transverse shear strains at both the top and bottom surfaces of the structures without the need for shear correction factors. The true drilling degrees of freedom of the proposed flat shell element are achieved using Allman-type in-plane displacement approximations. To smooth the membrane and bending strains, enhance strain gradients, and explicitly compute stiffness matrices for both plates and shells, we combine a cell-based smoothed technique with cubic shape functions at a bubble node, which is located at the centroid of the element. The proposed HSDT-type CS-MITC3 + flat shell element is applicable for both thick and thin plates and shells, utilizing the MITC3 + shear removal technique to re-interpolate transverse shear strains. The proposed flat shell element demonstrates excellent precision and convergence in analyses of porous plate and shell structures with different geometries, thicknesses, porosity distributions, porosity coefficients, and boundary conditions.</p>