Abstract: The actual application of the engineering project requires a tag to be placed in a special metal cavity, which gravely blocks the propagation of the electro-magnetic power between the reader and the tag. In order to guarantee the effective communication between the reader and tag, an innovative semi-circular spiral antenna, which works on 13.56 MHz and whose effective communication distance reaches up to 100 mm, is developed to enable the synchronous transmission of the power and data. By using finite-element method, the key features of the semi-circular spiral antenna and its design process are given out, which provides a practical engineering solution for the homogeneous problems.