The porous silicon nitride capillary wicks with high porosity, small pore size and narrow pore size distribution were prepared by micropore structure of β-Si3N4 long columnar crystal grains growth and lapping. The effects of sintering temperature, molding pressure and content of pore-forming agent on the pore parameters were studied. The thermal performances of porous capillary wick of loop heat pipe were tested. The results show that the effects of sintering temperature, molding pressure and content of pore-forming agent acting on regulation of the micropore structure have a correlation with the pore size distribution. The diameter of β-Si3N4 columnar crystal grain decreases and the length-diameter ratio increases with the increase of porosity. As a result, the β-Si3N4 columnar crystal grain lapped skeleton tends to be fine and dense, which plays the role of dividing intergranular pores, offsets the increase of pore size caused by the increase of porosity, and realizes the synergy of small pore size and high porosity in a certain range. The regulation of pore parameters of porous silicon nitride is different from those of conventional materials prepared by particle stacking and sintering method, whose pore size increases with the increase of porosity. When the porosity of the typical porous wick materials is 54.0%, the permeability and average porosity of porous silicon nitride are about 6.5×10-14 m2 and 0.3 μm, respectively, more than 95% pore sizes are 0.1～0.4 μm, the maximum capillary force is up to 62 kPa, and the heat load power is over 200 W.