The superconducting outsert of the 40 T hybrid-magnet in High Magnetic Field Laboratory (HFML) of Chinese Academy of Sciences (CAS) requires a highly stabilized low- voltage high-current power supply. One of the design is a switching power supply with active DC power filter. As the current flowing through a Shunt Active Power Filter (SAPF) is only the ripple current, the power loss caused by the filter can be very low, so SAPF is more and more applied in high-current devices. Usually, a shunt PWM converter can be used to generate current which is equal but oppositely phased to the ripple current. However, EMI noises caused by PWM have great influence on the circuit and the speed of response is limited by the switching frequency. Meanwhile, the load voltage variation makes it very difficult to design parameters of the filter. In this paper, a novel shunt liner active power filter (SLAPF) is designed to overcome the disadvantages of existing dc active filter schemes. Structurally, SLAPF can be divided into two components, one is the current compensator, and the other is the auxiliary power supply. The current compensator is composed of a power supply with fixed output voltage and two parallel branches, each of which consists an adjusting transistor. The auxiliary power supply is used to ensure the voltage across the adjusting transistors keep in a proper range, so the transistors can always operate in the saturation region. Based on the on-resistance variability of adjusting transistors, just as the series linear regulator, the output current of the current compensator can accurately track the oppositely phased out ripple current of main circuit, which leads to a good filter effect. In the paper, the detail structure and working principle of SLAPF are presented at first, then the design flow is described, which includes the control method of the current compensator, the analysis of topology chosen for auxiliary power supply. At last, the experimental results show that the filtering effect of SLAPF is good enough to make the power supply meet the technical requirements.