Ranging ability valuation for high-repetition laser ranging system (High-repetition LRS) is essential in space debris measurement. Traditional approaches are usually based on system parameters, such as laser transmission frequency, single pulse energy, transmission aperture, etc. However, precision is not properly evaluated when system conditions varies from ideal conditions. In this paper, a degradation model with a new decrement process based on precision curves is proposed. First, the equation of detection probability and ranging distance is derived. Then, a degradation model to get detection-probability-dependent precision curves is established——a decrement process of data sparsity is simulated, by gradually decreasing effective data while keeping all the noisy data remained. From the curve, a detection probability “threshold” can be seen, from which the ranging ability of the LRS system can be evaluated. In this degradation model, the demanded ranging precision is satisfied automatically. The novel method is reliable and simple with only the detection-probability-dependent precision curve needed. Degradation modeling based on high-repetition ranging experiments for space debris——NORAD 10861, 11574, 16182, 25530 shows an improvement by about 20% in ranging ability evaluation without sacrificing the ranging precision.