The invention discloses a double-capillary-core evaporator applied to a flat-type LHP system. The double-capillary-core evaporator comprises a main body, a lower end cover, an upper end cover, a lower capillary core, an upper capillary core and a wire mesh. The upper end cover and the lower end cover are fixedly arranged on the two end faces of the main body respectively, and a plurality of through grooves are respectively formed in the upper end cover and the lower end cover to form an upper steam channel and a lower steam channel. The upper capillary core and the lower capillary core are contained in closed space and are in close contact with the end covers respectively; the capillary cores are sealed through the wire mesh to define a compensation cavity, a steam cavity is formed by surplus space, located on one side of the compensation cavity, in the closed space, the steam cavity is communicated with a condensing device through a steam outlet, the compensation cavity is communicated with the outside through a backflow liquid pipe, and after steam flowing out from the steam channels is gathered to the steam cavity, the steam is condensed in the outside and flows into the compensation cavity from the backflow liquid pipe to compensate for working media in the upper capillary core and the lower capillary core. According to the double-capillary-core evaporator, the heat transmission capacity of the LHP system can be improved, influences of lateral wall heat conduction on the compensation cavity are reduced, the operation temperature of the LHP system is lowered, and the operation performance of the LHP system is improved.