Defrosting operation of an air source heat pump unit is always started with a pre-set time, corresponding to a fixed mass of frost accumulation. The exact defrosting initiation time is hard to be quantitatively given due to dynamic and uneven frosting conditions, and thus resulting in series of mal-defrosting phenomena. For an air source heat pump unit with a multi-circuit outdoor coil, when the melted frost locally drained during defrosting was considered, the optimization of start defrosting control strategy becomes more complicated. Here, this fundamental problem was experimentally investigated, with frost nearly evenly distributed on the surface of the outdoor coil at the start of defrosting. Defrosting performance of the experimental air source heat pump unit at different frost accumulations with the melted frost local drainage were then comparatively analyzed. These measured and calculated physical parameters include the temperature of tube surface and melted frost, compressor suction and discharge pressures and their difference, thermal energy taken from indoor air and electricity inputs on compressor and fans during defrosting, etc. Results showed that, the defrosting efficiency reached its peak at 51.80% when frost accumulation was at 933 g. Thus, the time-based start defrosting control strategy was demonstrated to be optimized with this method. Contributions of this study could be used for adjusting the control strategy of air source heat pump units, which are also benefit for energy saving in their industry and residential applications. (C) 2018 Elsevier B.V. All rights reserved.