The wound core with ultrathin magnetic strip is expected to improve the high-frequency performance of the electronic devices in efficiency and scale. However, the analysis of magnetic properties and calculation of the magnetic loss, including core loss and air-gap loss for cutting double C-type core, are complex and have not been entirely solved. In this article, the magnetic properties of a C-type amorphous core (AMCC0025) at different air-gap lengths are measured through a newly constructed tester. The measurement results are analyzed in detail, and the mechanism of magnetic loss influenced by the air gap is revealed by quantifying the reluctance of the C-type core. An accurate core loss model considering a fringing effect is derived, together with another additional loss model is realized by regression analysis. The evaluation results show that the average error for the partial total loss is less than 10%, and the average error for additional loss and core loss is within 5%. All errors meet the requirements of engineering calculation.