The traditional quadratic boost converter has the high step-up ability, but it needs high voltage and large current stress, which increases the volume and the cost. Based on the traditional boost converter and a high step-up three-terminal network, an improved quadratic boost converter was presented. The converter's operation principle and its voltage and current stress of each switch were analyzed when it worked in continuous conduction mode(CCM). The result shows that the presented converter has not only higher step-up ability, but lower voltage and current stress in the case of same output voltage. At last, simulation proved the converter's correctness and feasibility.