The application of magnetic materials is of great significance to the development of cryocoolers. At present, the lack of thermal properties of magnetic materials at temperatures below 80 K severely limits the design and optimization of cryocooler regenerators. A low-temperature apparent thermal conductivity measurement apparatus for granular magnetic materials has been developed. The apparent thermal conductivity of Er3Ni under various pressures of helium in the 4-40 K temperature range has been measured, and then its thermal conduction factor has been calculated. The test results in different temperature ranges show that the apparent thermal conductivity of Er3Ni particles is 0.11-0.22 W/m·K under vacuum, corresponding to the thermal conduction factor of 0.31-0.53. When the pressure increases to 1.4-2.2 MPa, the apparent thermal conductivity tends to a definite value of 3 W/m·K, corresponding to the thermal conduction factor of 7. Furthermore, the heat leakage characteristics of the Er3Ni regenerator under different operating conditions have been studied, and a mixed filling scheme of particles with wire mesh has been proposed to reduce the axial heat leakage between the cold and hot ends of the regenerator. The results show that the cooling performance can be improved after the nylon mesh and 316 L stainless steel mesh are filled in the Er3Ni regenerator, and the reduction of heat leakage under the pressure of 1.6 MPa helium is as high as 12% and 8%, respectively.