Abstract: To improve the utilization efficiency of mushroom residues, Pennisetum sp.（Laimu No. 1）seedlings were used as the experimental material. Seven treatments with different amounts of Pleurotus ostreatus spent substrate were set up: CK（0 g·kg⁻¹）, SMS1（10.0 g·kg⁻¹）, SMS2（25.0 g·kg⁻¹）, SMS3（50.0 g·kg⁻¹）, SMS4（75.0 g·kg⁻¹）, SMS5（100.0 g·kg⁻¹）, and SMS6（150.0 g·kg⁻¹）. The effects on the growth of Laimu No. 1 seedlings were studied. The results showed that after 20 days of growth, the SMS4 treatment group performed the best, with a plant height of 26.9 cm, which was significantly different from the other treatments. After 50 days of growth, the stem diameter of the SMS4 treatment group increased by 35.25% compared to the CK control group, reaching 16.50 mm, and the root length was 92.96 cm, superior to other treatment groups. Additionally, after 45 days of growth, the chlorophyll content and net photosynthetic rate in the SMS4 treatment group were the highest. Therefore, adding mushroom residue to the soil is beneficial for the growth of Laimu No. 1 seedlings. The effect was most significant when the dosage reached 75.0 g·kg⁻¹.