A current transformer and a direct current source based on a current transformer are disclosed. In an embodiment, the current transformer includes two output ends; a first winding and a second winding connected in series between the two output ends; a main core; a bypass core, arranged to be magnetically coupled with the main core. The first winding is wound on a part of the main core and a part of the bypass core, and the second winding is wound on apart of the bypass core. In an embodiment, the current transformer also includes a high-frequency bypass, connected in parallel with the first winding, and used to filter a high-frequency signal. The high-frequency bypass provides a low-impedance path for a high-frequency signal in a primary conductor under measurement, such that the bypass core with the second winding presents less obstruction to the main core, thereby reducing heating.