In this paper, a numerical investigation of a humidification-dehumidification (HDH) desalination system driven by heat pump is presented. The presented system consists of heat pump unit, plate heat exchanger, humidifier, first stage dehumidifier and second stage dehumidifier. A mathematical model based on the heat and mass balance equations of each component was built and verified by the experimental results. The effects of several important parameters of air and seawater into the system on system performance were numerically studied. The parametric analysis indicates that the air temperature has insignificant influence on the system yield, the increment of air humidity ratio or seawater flow rate is obviously beneficial for the productivity improvement and the productivity has different trends with increasing seawater temperature. It is also revealed that the productivity firstly increases and then decreases with the increment of air flow rate. Furthermore, gained output ratio (GOR) of the proposed system and the cost of fresh water are estimated. The average values of GOR and cost are 2.532 and 0.0412 USA dollar (USD) per kilogram respectively, which are competitive in comparison with other reported HDH systems. Additionally, a numerical correlation for the system productivity was regressed with an acceptable deviation.