This paper presents our development of the design support system for Ocean Thermal Energy Conversion (OTEC) and Deep Ocean Water utilization plant such as seawater desalination, district air conditioning, aquaculture, ocean fertilization, etc.. As this plant is characterized by multi-purpose complex unlike other renewable energy facilities, its planning and initial designing work becomes tangled process with various design candidates of plant configuration each having a lot of design parameters and constraints. Under these circumstances, we have been developing design support system focusing on how to support defining configuration models effectively and establishing better design procedure to decide design parameters.