Four lactic acid bacteria (LAB) strains isolated from naturally ensiled broccoli waste were characterized, and their effects on the fermentation quality of high-moisture broccoli waste silage were studied. The four isolated strains were assessed using the morphological, physiological and biochemical tests. The four strains were added to broccoli waste at three moisture contents (90%, W0; 87%, W1; 80%, W2) and ensiled for 60 days. All strains (CB89, CB94, CB112, and CB120) grew at 15–45 °C, pH 3.0–7.0, and in 3.0–6.5% NaCl and were identified as Lactiplantibacillus plantarum by 16S rDNA sequencing. Inoculation of CB120 significantly (p < 0.05) increased lactic acid, starch, and non-structural carbohydrate content, and significantly (p < 0.05) decreased pH values and aerobic bacteria count compared with control (CK) at all three moisture contents. In conclusion, CB120 improved the fermentation quality and nutritional value of broccoli waste silage at three moisture contents and could be applied as a promising additive for high-moisture material.