A new scheme of serially concatenated systematic polar codes is presented. In this coding, the low reliable message bits of the outer polar code are re-encoded by the inner polar code. This is because bits recognised as low reliable ones are more vulnerable to the channel noise. A spectrum distance method is analyzed to compare the performance of the proposed code with previously constructed ones. In addition, the existence of two constituent codes, which unequally protect information bits, provides unequal error protection (UEP) capability for the concatenated code. Conducted analysis and simulations confirm that with same rate and code length, the newly designed codes outperform the existing punctured polar codes.