In response to the issue of reducing satellite constellation backup costs while meeting constellation availability metrics. This paper studies the impact of different spare strategies on the performance of a Low Earth Orbit satellite constellation and select the best spare strategy that takes into account both the performance and economic efficiency of the constellation. First and foremost, a constellation availability model with degraded operation is developed. Secondly, the Markov process is used to solve this model. Last but not least, the availability of the constellation with different spare strategies at different constellation levels is calculated based on the operation data of a real LEO satellite constellation. The results of the analysis prove that deploying two backup satellites in each orbit can make the constellation with the lowest spare cost while meeting the availability requirement. The spare strategy proposed in this paper has a certain reference value for the construction of LEO satellite constellations and can provide guidance for the accurate configuration of spare satellites with reasonable cost reduction.