The detailed numerical simulation of the unsteady drag coefficient of deformable droplet is investigated with a mass conserving level set (LS) method. The simulation results indicate that the unsteady drag coefficient is always larger than the steady standard drag coefficient for the present decelerating relative flow. The effects of Weber number, density ratio and viscosity ratio on the unsteady drag coefficient of drop deformation are studied. It is found that the unsteady term (including density ratio) has the most effect on the unsteady drag coefficient and Weber number secondly. The viscosity ratio has little effect on the unsteady drag coefficient due to low Ohnesorge number. The numerical results confirm that the difference between drag coefficient and standard steady drag coefficient has an approximately linear curve fit with the unsteady parameter for low Weber number. These results will lay the foundation for the modeling of unsteady drag coefficient in our further work. (C) 2016 Elsevier B.V. All rights reserved.