The invention relates to an evaporator of a loop heat pipe and a preparation method of the evaporator. The evaporator comprises an outer cylindrical wall, a heat absorption face, an upper end cover, a capillary core and a vapor-liquid isolation barrel with openings in the two ends. A cavity structure is formed by the outer barrel wall, the heat absorption face and the upper end cover, the capillary core is attached to the heat absorption face, one end of the vapor-liquid isolation barrel is in contact with the surface of the capillary core, the capillary core is pressed on the heat absorption face, and the other end of the vapor-liquid isolation barrel is fixedly connected with the upper end cover; the cavity structure formed by the outer barrel wall, the heat absorption face and the upper end over is divided into two parts through the vapor-liquid isolation barrel, a vapor cavity is formed in the vapor-liquid isolation barrel, and a liquid cavity is formed outside the vapor-liquid isolation barrel. The evaporator is ingenious in structural design and excellent in performance, it is effectively guaranteed that vapor and liquid in the evaporator are separated, certain positive pressure and negative pressure can be borne, that is, a coupling face is not likely to deform, the machining method is simple and reliable, and volume production is convenient.