The small-scale flat capillary pumped loop（CPL）for the cooling of electronic apparatus was presented，and its operation principle and characteristics were analyzed. Based on the structure characteristics of small-scale flat CPL evaporator，the effect of metal side wall conduction of evaporator on the CPL work limit was analyzed，and the heat transfer limit of side wall effect was introduced. In order to improve the heat transfer capacity of CPL，fins were added on the bottom of evaporator. The flow and heat transfer in the porous structure were modeled by using two-phase layered saturated porous model，and the conduction models of metal wall and fluid region were established. The entire evaporator was solved with the SIMPLE algorithm as a conjugate problem. The numerical results showed that the heated surface temperatures of the evaporator were low，and the temperature difference along the heated surface was small. The temperature gradients in the wick structure of evaporator with fins increased，and the temperature distribution in the wick structure was reasonable. The temperatures of heated surface and bottom metal wall of finned evaporator decreased，and the heat transfer capacity was improved remarkably.