Based on the oblique import inlet of 45° coupling with straight cavity, four inlets with intake open and close coupling 5 mm×5 mm grille and 15 mm×15 mm grille are designed. The numerical simulation is carried out on electromagnetic scattering characteristics of grille based on multilevel fast multipole method (MLFMM). And then the same inlet test models are made to verify the method for electromagnetic scattering calculation. Finally, the typical geometric parameters of electromagnetic scattering characteristics such as the grille hole spacing, grille inclination angle, and grille thickness are obtained. Results indicate that: (1)The calculated result of electromagnetic scattering characteristics of grille is close to the experimental data at the azimuth angle from -30° to 30°, indicating the validity of numerical simulation method. (2) At the frequency of 10 GHz, the electromagnetic shielding effectiveness can reach about 43% when the grille hole spacing is λ/2(λ represents wavelength), and it will be shielding entirely while the grille hole spacing achieves λ/6. (3) With the increasing of the grille inclination angle, the electromagnetic shielding effectiveness reduces gradually. (4)With the increasing of the grille thickness increasing, the electromagnetic shielding effectiveness also increases gradually, but the increasing extent decreases.