The Chinese government’s program of reverting agriculture to forests and grasslands has largely been responsible for the phenomenon of the Loess Plateau becoming green, which has attracted the attention of the international academic community. To identify the reasons for the Loess Plateau’s greening scientifically and precisely, it is essential to comprehend the relationship between the natural vegetation covering index and precipitation change. The relationship between NDVI indicators and precipitation in the non-reforestation (grass) area of the Loess Plateau was investigated using statistical techniques like detrended partial cross-correlation analysis and observation data of daily precipitation and pan evaporation in the Loess Plateau from 1982 to 2015. Also looked at are the results of this co-evolution of space and time. The results demonstrate that during the process of becoming greener, the NDVI index in the non-reforestation (grass) portion of the Loess Plateau showed a significant rising trend. The most noticeable shift happened in the hilly and gully areas. The yearly precipitation data often displayed a fluctuating, increasing tendency. It was discovered to have a strong long-range correlation with the NDVI data. The positive partial correlation area accounted for about 76% of the total area of the Loess Plateau. There is regional variation in the effect of different magnitudes of precipitation changes on the NDVI in the non-reforestation (grass) regions of the Loess Plateau, and 30–50 mm precipitation increases are the key to the greatly increased natural NDVI in these places. Thus, it is determined that precipitation variations have a significant impact on the greening of the Loess Plateau, particularly in non-reforestation (grass) areas.