The basic contents of electromagnetic compatibility, interference and shielding are introduced briefly. The shielding effectiveness of the chassis cover and the calculation method of shielding effectiveness of the chassis connection gap are discussed. The main factors influencing the shielding effectiveness of the chassis are analyzed. The main measures to improve the shielding effectiveness of the chassis are proposed. Based on an analysis of a control chassis, the reliability of the measures improving the shielding effectiveness of the chassis gap is verified. In the design of electromagnetic shielding, it is necessary to comprehensively consider the various influencing factors which can influence the electromagnetic shielding, so a better shielding effect can be achieved.