In the Saharan regions of Algeria, the phenomenon of sand encroachment is the main cause of the degradation of the physical environment as well as the biological resources. It represents an impediment against all forms of socio-economic activities. In the past, fighting sand encroachment in the region of Gourara (southwest of Algeria) was carried out through ancestral processes like fences and small clay walls. Since 2007, this region has witnessed the plantation of several green belts, planted by the forest services as a part of the biological fight against this phenomenon. The present study evaluates the effectiveness of these green belts in the fight against sand encroachment by the application of a new index of sand stabilization (ISS). The calculation of this index is performed with two factors: the rate of recovery and the height of sand accumulations stabilized inside the green belts. In addition, the status of this index was determined by two parameters: the calculated index value and the field of observation. Thus, a positive validation of effectiveness of this index has been found after its application on different cases of green belts. The multivariate analysis (PCA) of 60 sand encroachment surveys on the three studied green belts (BVO, BVD, BVG) enabled to distinguish three groups of surveys, according to both the values recovery rate and the height of sand accumulations. This new index has allowed to represent the sand stabilization situation of each green belt. Thus, three situations were found: weak, average, and strong stabilization.