Medical nitric oxide (NO) flow control system plays an important role in lowering pulmonary hypertension. The design requirements, overall scheme, delivery system and hardware circuits of a medical NO flow control system were introduced in this paper. Particularly, we proposed the design of NO delivery system and hardware circuits in detail. To deliver nitric oxide of a variable concentration, the designed system needs to work with a ventilator. The system can adjust and monitor the inhaled nitric oxide concentrations and send out sound and light alarms when the inhaled nitric oxide concentrations are out of the set range. To validate reliability and efficacy, we measured specifications such as linearity, stability and response time of the proposed NO flow control system by continuously administering nitric oxide into inspiratory circuit to deliver nitric oxide of variable concentrations to a test lung. The experiments showed that these specifications can meet the desired requirements.