Constructal design of an evaporator in ocean thermal energy conversion system (OTECS) is carried out by taking the minimum dimensionless pumping power (DPP) as optimization objective. The effective volume and heat transfer rate (HTR) of the evaporator are taken as constraints in the constructal optimization. The optimal heat transfer plate (HTP) width of the evaporator is obtained. The effects of the structure and flow parameters and types of working fluid on the minimum DPP and optimal HTP width are analyzed. The results show that the DPP after constructal optimization reduces by 37.1% compared with that of the initial design point. The minimum DPP dramatically augments with the increases of HTP effective length, corrugation angle and mass flow rate of the warm seawater, and dramatically diminishes with the increases of corrugation wavelength and effective volume of the evaporator. The influence degree to optimal results varies for different design parameters and working fluids. The results can offer some valuable guidelines for the optimal designs and applications of the evaporators in OTECSs.