Objective under noninvasive positive pressure ventilation, in view of the inconvenient operations in the airway of human, and avoiding the interference of the spontaneous breathing and the necessary leakage flow, this paper devoted much attention on the method in getting the respiratory resistance (R) and compliance (C) on line. Method At the end of expiration, when the exhaled flow is 0, relative to the expiration positive airway pressure (EPAP), a step-dropped pressure is produced with the amplitude of (ΔP) and hold time of Δt. Under this dropped pressure, a discharge flow is generated from lung. In addition, a respiratory model was developed in MATLAB to simulate the breaths of normal adult, ARDS patient and COPD patient. With collecting the data of airflow in these simulation experiments, the corresponding R and C is calculated. Results The calculated R and C, for normal adult, ARDS patient, COPD patient, respectively deviate from the actual value with 1.60% and -1.60%, -8.60 and 10%, -3.75% and 4.00%. Conclusions The proposed algorithm is practicable and feasible for calculating the respiratory R and C. The implementation of this algorithm in simulation will be a strong basis for further researches and application. This algorithm is beneficial to study the intelligent ventilation and proportional assist ventilation in respirator.