The characteristics of the surface inactive layer of a 1-kg-mass p-type point-contact germanium detector were studied. The thickness of the inactive layer and its uniformity on the top and lateral surfaces were measured. A charge collection efficiency function was developed according to the Monte Carlo simulation to describe the charge collection capacity along the depth within this inactive layer. In the energy range below 18 keV, the surface, bulk, and total spectra of ⁵⁷Co, ¹³³Ba, ¹³⁷Cs, and ⁶⁰Co from simulations based on the charge collection efficiency function were well consistent with those from experiments.