The loop heat pipe (LHP) is a passive heat sink used in aerospace and electronic devices. As the core component of the LHP, the physical property parameters of porous wick directly affect the overall performance of the LHP. In this paper, the performance of the porous wick is improved by adjusting the pore size, thereby improving the performance of the LHP. The nickel-based double-pore porous wicks are prepared by T225 nickel powder and NaCl particles, and the pore size of the porous wicks can be changed by different cold pressing force (30 kN, 40 kN, 50 kN, and 60 kN). The effects of different cold pressing force on the porosity, permeability, and other physical property parameters are studied when the ratio of pore former is 20 wt.%. In the end, we select the cold pressing force of 30 kN to prepare the porous wick of the LHP. Then the effects of constant load and variable load of the heat transfer performance under different placement elevations are studied. The results show that the heat load range is 10 W–100 W, the minimum evaporator thermal resistance is 0.424 K/W, and the minimum LHP thermal resistance is 0.598 K/W. When β = 0 deg, there is a “backflow” phenomenon at the initial stage of low thermal load. With the increase of thermal load, the “backflow” duration decreases until it disappears, and the startup time becomes shorter. The thermal resistances of the evaporator and LHP decrease rapidly and then slowly increase. When β = –90 deg, the LHP appears to demonstrate “reverse startup” phenomenon.