The invention belongs to the field of nuclear power unit design, in particular to a loose constraint genetic simplex algorithm based optimal design method for a condenser. The method comprises the following steps: establishing a parameter evaluation model of the condenser; determining optimal variables, an objective function and a constraint function, and establishing an optimization model of the condenser; generating an initial population according to the optimization model; creating a loose restraint processing method for evaluating the individual quality; carrying out the genetic algorithm optimization of improved optimization strategy on the population, and generating a new population obtained after genetic evolution; generating the next generation of new population; utilizing the loose constraint processing method to evaluate the next generation of new population, and updating the restraint looseness; if meeting the termination condition, terminating the population evolution, and outputting the global optimal solution. The invention provides the loose restraint processing method for the complex nonlinear constraint problem of condenser optimization, and the loose restraint processing method is integrated into a hybrid optimization algorithm, so that the ability of optimization of the algorithm is greatly enhanced, and the effectiveness of condenser optimization design is ensured.