During flow boiling, the heat transfer coefficient increases with the increasing of vapor quality, and the highly efficient heat transfer exists in the high vapor quality or annular flow region. The evaporator with active vapor-liquid adjustment (AE) uses the separators to redistribute the vapor quality and mass flux. In this way, the efficient heat transfer could be repeated in the tubes, improving heat transfer capacity and reducing pressure drop. A more practical question is raised as 'how much the performance can be improved and how much the heat transfer area can be saved by AE?' Targeting to these, a numerical model of AE is developed in this paper and verified by experimental data, with an error less than 20%. By implementing the multi-objective optimization algorithm, three optimal layouts, which have a significantly reduced pressure drop by 32.5% or an increased heat transfer capacity by 8.9% compared to the conventional evaporator (CE), are obtained. The main reason for their performance improvement is the efficient heat transfer region occupies more heat transfer area, which accounts for 2/3 of the total area. Moreover, to obtain the maximum reduced heat transfer area but having the equivalent performance with CE, the AE layout is reoptimized and the heat transfer area can be reduced by 21%. This paper gives interesting way from perspectives on enhancing the evaporator performance or reducing its heat transfer area.